0day-ci / linux

This patch ties together the work in the previous patches, to introduce
semantics around the qgroup creation / removal API that are a bit more
intuitive that the current one. It also creates two new args structures
which have reserved space for future expansion, as opposed to single
datastructure for both creation and removal.

The associated semantics are as follows:
1) You cannot create a level 0 qgroup for a subvol which does not exist
   unless you pass in an override flag.
2) You cannot delete a level 0 qgroup which refers to a subvolume, which
   currently exists unless you pass in an override flag.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

This patch is to prepare for following patches in this patchset. The code
allows you to check if a subvol exists, and to only allow the creation
of qgroups on subvols that already exist. It doesn't make sense to allow
the creation of level 0 qgroups otherwise.

The behaviour is to inherit (create) a qgroup when you create a new
subvol with quota on. If there is an existing qgroup with this same
ID, it will be reset.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

This patch is to prepare for following patches in this patchset. The
purpose is to make it so that we can prevent accidental removal of
qgroups that are actively in use.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

This patch is just changing whitespace alignment in qgroup.h

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

This change follows the change to automatically remove qgroups
if the associated subvolume has also been removed. It changes
the default behaviour to automatically remove qgroups when
a subvolume is deleted, but this can be override with the
qgroup_keep mount flag.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

This change is purely a style change, so that btrfs_create_qgroup is
split. There is some whitespace changes, but this shouldn't have any
effect on the logic of the code.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

Previously, we were calling del_qgroup_item, and ignoring the return code
resulting in a potential to have divergent in-memory state without an
error. Perhaps, it makes sense to handle this error code, and put the
filesystem into a read only, or similar state.

This patch only adds reporting of the error.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

This change is purely a style change, so that btrfs_remove_qgroup is
split. There is some whitespace changes, but this shouldn't have any
effect on the logic of the code.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

The code to fetch a 64-bit value from user space was entirely buggered,
and has been since the code was merged in early 2016 in commit
b2f6803 ("x86/mm/32: Add support for 64-bit __get_user() on 32-bit
kernels").

Happily the buggered routine is almost certainly entirely unused, since
the normal way to access user space memory is just with the non-inlined
"get_user()", and the inlined version didn't even historically exist.

The normal "get_user()" case is handled by external hand-written asm in
arch/x86/lib/getuser.S that doesn't have either of these issues.

There were two independent bugs in __get_user_asm_u64():

 - it still did the STAC/CLAC user space access marking, even though
   that is now done by the wrapper macros, see commit 11f1a4b
   ("x86: reorganize SMAP handling in user space accesses").

   This didn't result in a semantic error, it just means that the
   inlined optimized version was hugely less efficient than the
   allegedly slower standard version, since the CLAC/STAC overhead is
   quite high on modern Intel CPU's.

 - the double register %eax/%edx was marked as an output, but the %eax
   part of it was touched early in the asm, and could thus clobber other
   inputs to the asm that gcc didn't expect it to touch.

   In particular, that meant that the generated code could look like
   this:

        mov    (%eax),%eax
        mov    0x4(%eax),%edx

   where the load of %edx obviously was _supposed_ to be from the 32-bit
   word that followed the source of %eax, but because %eax was
   overwritten by the first instruction, the source of %edx was
   basically random garbage.

The fixes are trivial: remove the extraneous STAC/CLAC entries, and mark
the 64-bit output as early-clobber to let gcc know that no inputs should
alias with the output register.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: stable@kernel.org   # v4.8+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Al noticed that unsafe_put_user() had type problems, and fixed them in
commit a7cc722 ("fix unsafe_put_user()"), which made me look more
at those functions.

It turns out that unsafe_get_user() had a type issue too: it limited the
largest size of the type it could handle to "unsigned long".  Which is
fine with the current users, but doesn't match our existing normal
get_user() semantics, which can also handle "u64" even when that does
not fit in a long.

While at it, also clean up the type cast in unsafe_put_user().  We
actually want to just make it an assignment to the expected type of the
pointer, because we actually do want warnings from types that don't
convert silently.  And it makes the code more readable by not having
that one very long and complex line.

[ This patch might become stable material if we ever end up back-porting
  any new users of the unsafe uaccess code, but as things stand now this
  doesn't matter for any current existing uses. ]

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

…/git/viro/vfs

Pull misc uaccess fixes from Al Viro:
 "Fix for unsafe_put_user() (no callers currently in mainline, but
  anyone starting to use it will step into that) + alpha osf_wait4()
  infoleak fix"

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  osf_wait4(): fix infoleak
  fix unsafe_put_user()

…/linux/kernel/git/tip/tip

Pull scheduler fix from Thomas Gleixner:
 "A single scheduler fix:

  Prevent idle task from ever being preempted. That makes sure that
  synchronize_rcu_tasks() which is ignoring idle task does not pretend
  that no task is stuck in preempted state. If that happens and idle was
  preempted on a ftrace trampoline the machine crashes due to
  inconsistent state"

* 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched/core: Call __schedule() from do_idle() without enabling preemption

…inux/kernel/git/tip/tip

Pull irq fixes from Thomas Gleixner:
 "A set of small fixes for the irq subsystem:

   - Cure a data ordering problem with chained interrupts

   - Three small fixlets for the mbigen irq chip"

* 'irq-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  genirq: Fix chained interrupt data ordering
  irqchip/mbigen: Fix the clear register offset calculation
  irqchip/mbigen: Fix potential NULL dereferencing
  irqchip/mbigen: Fix memory mapping code

failing sys_wait4() won't fill struct rusage...

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

__put_user_size() relies upon its first argument having the same type as what
the second one points to; the only other user makes sure of that and
unsafe_put_user() should do the same.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

…nel/git/rostedt/linux-trace

Pull tracing fixes from Steven Rostedt:

 - Fix a bug caused by not cleaning up the new instance unique triggers
   when deleting an instance. It also creates a selftest that triggers
   that bug.

 - Fix the delayed optimization happening after kprobes boot up self
   tests being removed by freeing of init memory.

 - Comment kprobes on why the delay optimization is not a problem for
   removal of modules, to keep other developers from searching that
   riddle.

 - Fix another case of rcu not watching in stack trace tracing.

* tag 'trace-v4.12-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
  tracing: Make sure RCU is watching before calling a stack trace
  kprobes: Document how optimized kprobes are removed from module unload
  selftests/ftrace: Add test to remove instance with active event triggers
  selftests/ftrace: Fix bashisms
  ftrace: Remove #ifdef from code and add clear_ftrace_function_probes() stub
  ftrace/instances: Clear function triggers when removing instances
  ftrace: Simplify glob handling in unregister_ftrace_function_probe_func()
  tracing/kprobes: Enforce kprobes teardown after testing
  tracing: Move postpone selftests to core from early_initcall

Pull block fixes from Jens Axboe:
 "A small collection of fixes that should go into this cycle.

   - a pull request from Christoph for NVMe, which ended up being
     manually applied to avoid pulling in newer bits in master. Mostly
     fibre channel fixes from James, but also a few fixes from Jon and
     Vijay

   - a pull request from Konrad, with just a single fix for xen-blkback
     from Gustavo.

   - a fuseblk bdi fix from Jan, fixing a regression in this series with
     the dynamic backing devices.

   - a blktrace fix from Shaohua, replacing sscanf() with kstrtoull().

   - a request leak fix for drbd from Lars, fixing a regression in the
     last series with the kref changes. This will go to stable as well"

* 'for-linus' of git://git.kernel.dk/linux-block:
  nvmet: release the sq ref on rdma read errors
  nvmet-fc: remove target cpu scheduling flag
  nvme-fc: stop queues on error detection
  nvme-fc: require target or discovery role for fc-nvme targets
  nvme-fc: correct port role bits
  nvme: unmap CMB and remove sysfs file in reset path
  blktrace: fix integer parse
  fuseblk: Fix warning in super_setup_bdi_name()
  block: xen-blkback: add null check to avoid null pointer dereference
  drbd: fix request leak introduced by locking/atomic, kref: Kill kref_sub()

On rdma read errors, release the sq ref that was taken
when the req was initialized. This avoids a hang in
nvmet_sq_destroy() when the queue is being freed.

Signed-off-by: Vijay Immanuel <vijayi@attalasystems.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

Remove NVMET_FCTGTFEAT_NEEDS_CMD_CPUSCHED. It's unnecessary.

Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

Per the recommendation by Sagi on:
http://lists.infradead.org/pipermail/linux-nvme/2017-April/009261.html

Rather than waiting for reset work thread to stop queues and abort the ios,
immediately stop the queues on error detection. Reset thread will restop
the queues (as it's called on other paths), but it does not appear to have
a side effect.

Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

In order to create an association, the remoteport must be
serving either a target role or a discovery role.

Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

FC Port roles is a bit mask, not individual values.
Correct nvme definitions to unique bits.

Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

CMB doesn't get unmapped until removal while getting remapped on every
reset. Add the unmapping and sysfs file removal to the reset path in
nvme_pci_disable to match the mapping path in nvme_pci_enable.

Fixes: 202021c ("nvme : Add sysfs entry for NVMe CMBs when appropriate")

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Reviewed-By: Stephen Bates <sbates@raithlin.com>
Cc: <stable@vger.kernel.org> # 4.9+
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

…rnel/git/gregkh/staging

Pull staging driver fixes from Greg KH:
 "Here are a number of staging driver fixes for 4.12-rc2

  Most of them are typec driver fixes found by reviewers and users of
  the code. There are also some removals of files no longer needed in
  the tree due to the ion driver rewrite in 4.12-rc1, as well as some
  wifi driver fixes. And to round it out, a MAINTAINERS file update.

  All have been in linux-next with no reported issues"

* tag 'staging-4.12-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging: (22 commits)
  MAINTAINERS: greybus-dev list is members-only
  staging: fsl-dpaa2/eth: add ETHERNET dependency
  staging: typec: fusb302: refactor resume retry mechanism
  staging: typec: fusb302: reset i2c_busy state in error
  staging: rtl8723bs: remove re-positioned call to kfree in os_dep/ioctl_cfg80211.c
  staging: rtl8192e: GetTs Fix invalid TID 7 warning.
  staging: rtl8192e: rtl92e_get_eeprom_size Fix read size of EPROM_CMD.
  staging: rtl8192e: fix 2 byte alignment of register BSSIDR.
  staging: rtl8192e: rtl92e_fill_tx_desc fix write to mapped out memory.
  staging: vc04_services: Fix bulk cache maintenance
  staging: ccree: remove extraneous spin_unlock_bh() in error handler
  staging: typec: Fix sparse warnings about incorrect types
  staging: typec: fusb302: do not free gpio from managed resource
  staging: typec: tcpm: Fix Port Power Role field in PS_RDY messages
  staging: typec: tcpm: Respond to Discover Identity commands
  staging: typec: tcpm: Set correct flags in PD request messages
  staging: typec: tcpm: Drop duplicate PD messages
  staging: typec: fusb302: Fix chip->vbus_present init value
  staging: typec: fusb302: Fix module autoload
  staging: typec: tcpci: declare private structure as static
  ...

…/git/gregkh/usb

Pull USB fixes from Greg KH:
 "Here are a number of small USB fixes for 4.12-rc2

  Most of them come from Johan, in his valiant quest to fix up all
  drivers that could be affected by "malicious" USB devices. There's
  also some fixes for more "obscure" drivers to handle some of the
  vmalloc stack fallout (which for USB drivers, was always the case, but
  very few people actually ran those systems...)

  Other than that, the normal set of xhci and gadget and musb driver
  fixes as well.

  All have been in linux-next with no reported issues"

* tag 'usb-4.12-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb: (42 commits)
  usb: musb: tusb6010_omap: Do not reset the other direction's packet size
  usb: musb: Fix trying to suspend while active for OTG configurations
  usb: host: xhci-plat: propagate return value of platform_get_irq()
  xhci: Fix command ring stop regression in 4.11
  xhci: remove GFP_DMA flag from allocation
  USB: xhci: fix lock-inversion problem
  usb: host: xhci-ring: don't need to clear interrupt pending for MSI enabled hcd
  usb: host: xhci-mem: allocate zeroed Scratchpad Buffer
  xhci: apply PME_STUCK_QUIRK and MISSING_CAS quirk for Denverton
  usb: xhci: trace URB before giving it back instead of after
  USB: serial: qcserial: add more Lenovo EM74xx device IDs
  USB: host: xhci: use max-port define
  USB: hub: fix SS max number of ports
  USB: hub: fix non-SS hub-descriptor handling
  USB: hub: fix SS hub-descriptor handling
  USB: usbip: fix nonconforming hub descriptor
  USB: gadget: dummy_hcd: fix hub-descriptor removable fields
  doc-rst: fixed kernel-doc directives in usb/typec.rst
  USB: core: of: document reference taken by companion helper
  USB: ehci-platform: fix companion-device leak
  ...

…kernel/git/gregkh/char-misc

Pull char/misc driver fixes from Greg KH:
 "Here are five small bugfixes for reported issues with 4.12-rc1 and
  earlier kernels. Nothing huge here, just a lp, mem, vpd, and uio
  driver fix, along with a Kconfig fixup for one of the misc drivers.

  All of these have been in linux-next with no reported issues"

* tag 'char-misc-4.12-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc:
  firmware: Google VPD: Fix memory allocation error handling
  drivers: char: mem: Check for address space wraparound with mmap()
  uio: fix incorrect memory leak cleanup
  misc: pci_endpoint_test: select CRC32
  char: lp: fix possible integer overflow in lp_setup()

Pull watchdog fixes from Wim Van Sebroeck:
 - orion_wdt compile-test dependencies
 - sama5d4_wdt: WDDIS handling and a race confition
 - pcwd_usb: fix NULL-deref at probe
 - cadence_wdt: fix timeout setting
 - wdt_pci: fix build error if SOFTWARE_REBOOT is defined
 - iTCO_wdt: all versions count down twice
 - zx2967: remove redundant dev_err call in zx2967_wdt_probe()
 - bcm281xx: Fix use of uninitialized spinlock

* git://www.linux-watchdog.org/linux-watchdog:
  watchdog: bcm281xx: Fix use of uninitialized spinlock.
  watchdog: zx2967: remove redundant dev_err call in zx2967_wdt_probe()
  iTCO_wdt: all versions count down twice
  watchdog: wdt_pci: fix build error if define SOFTWARE_REBOOT
  watchdog: cadence_wdt: fix timeout setting
  watchdog: pcwd_usb: fix NULL-deref at probe
  watchdog: sama5d4: fix race condition
  watchdog: sama5d4: fix WDDIS handling
  watchdog: orion: fix compile-test dependencies

…airlied/linux

Pull drm fixes from Dave Airlie:
 "Mostly nouveau and i915, fairly quiet as usual for rc2"

* tag 'drm-fixes-for-v4.12-rc2' of git://people.freedesktop.org/~airlied/linux:
  drm/atmel-hlcdc: Fix output initialization
  gpu: host1x: select IOMMU_IOVA
  drm/nouveau/fifo/gk104-: Silence a locking warning
  drm/nouveau/secboot: plug memory leak in ls_ucode_img_load_gr() error path
  drm/nouveau: Fix drm poll_helper handling
  drm/i915: don't do allocate_va_range again on PIN_UPDATE
  drm/i915: Fix rawclk readout for g4x
  drm/i915: Fix runtime PM for LPE audio
  drm/i915/glk: Fix DSI "*ERROR* ULPS is still active" messages
  drm/i915/gvt: avoid unnecessary vgpu switch
  drm/i915/gvt: not to restore in-context mmio
  drm/etnaviv: don't put fence in case of submit failure
  drm/i915/gvt: fix typo: "supporte" -> "support"
  drm: hdlcd: Fix the calculation of the scanout start address

…it/jejb/scsi

Pull SCSI fixes from James Bottomley:
 "This is the first sweep of mostly minor fixes. There's one security
  one: the read past the end of a buffer in qedf, and a panic fix for
  lpfc SLI-3 adapters, but the rest are a set of include and build
  dependency tidy ups and assorted other small fixes and updates"

* tag 'scsi-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/jejb/scsi:
  scsi: pmcraid: remove redundant check to see if request_size is less than zero
  scsi: lpfc: ensure els_wq is being checked before destroying it
  scsi: cxlflash: Select IRQ_POLL
  scsi: qedf: Avoid reading past end of buffer
  scsi: qedf: Cleanup the type of io_log->op
  scsi: lpfc: double lock typo in lpfc_ns_rsp()
  scsi: qedf: properly update arguments position in function call
  scsi: scsi_lib: Add #include <scsi/scsi_transport.h>
  scsi: MAINTAINERS: update OSD entries
  scsi: Skip deleted devices in __scsi_device_lookup
  scsi: lpfc: Fix panic on BFS configuration
  scsi: libfc: do not flood console with messages 'libfc: queue full ...'

…ux/kernel/git/nvdimm/nvdimm

Pull libnvdimm fixes from Dan Williams:
 "A couple of compile fixes.

  With the removal of the ->direct_access() method from
  block_device_operations in favor of a new dax_device + dax_operations
  we broke two configurations.

  The CONFIG_BLOCK=n case is fixed by compiling out the block+dax
  helpers in the dax core. Configurations with FS_DAX=n EXT4=y / XFS=y
  and DAX=m fail due to the helpers the builtin filesystem needs being
  in a module, so we stub out the helpers in the FS_DAX=n case."

* 'libnvdimm-for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm:
  dax, xfs, ext4: compile out iomap-dax paths in the FS_DAX=n case
  dax: fix false CONFIG_BLOCK dependency

…kernel/git/wsa/linux

Pull i2c fix from Wolfram Sang:
 "A regression fix for I2C that would be great to have in rc2"

* 'i2c/for-current' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux:
  i2c: designware: don't infer timings described by ACPI from clock rate

…ux/kernel/git/joro/iommu

Pull IOMMU fixes from Joerg Roedel:

 - another compile-fix as a fallout of the recent header-file cleanup

 - add a missing IO/TLB flush to the Intel VT-d kdump code path

 - a fix for ARM64 dma code to only access initialized iova_domain
   members

* tag 'iommu-fixes-v4.12-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu:
  iommu/mediatek: Include linux/dma-mapping.h
  iommu/vt-d: Flush the IOTLB to get rid of the initial kdump mappings
  iommu/dma: Don't touch invalid iova_domain members

Pull KVM fixes from Radim Krčmář:
 "ARM:
   - a fix for a build failure introduced in -rc1 when tracepoints are
     enabled on 32-bit ARM.

   - disable use of stack pointer protection in the hyp code which can
     cause panics.

   - a handful of VGIC fixes.

   - a fix to the init of the redistributors on GICv3 systems that
     prevented boot with kvmtool on GICv3 systems introduced in -rc1.

   - a number of race conditions fixed in our MMU handling code.

   - a fix for the guest being able to program the debug extensions for
     the host on the 32-bit side.

  PPC:
   - fixes for build failures with PR KVM configurations.

   - a fix for a host crash that can occur on POWER9 with radix guests.

  x86:
   - fixes for nested PML and nested EPT.

   - a fix for crashes caused by reserved bits in SSE MXCSR that could
     have been set by userspace.

   - an optimization of halt polling that fixes high CPU overhead.

   - fixes for four reports from Dan Carpenter's static checker.

   - a protection around code that shouldn't have been preemptible.

   - a fix for port IO emulation"

* tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm: (27 commits)
  KVM: x86: prevent uninitialized variable warning in check_svme()
  KVM: x86/vPMU: fix undefined shift in intel_pmu_refresh()
  KVM: x86: zero base3 of unusable segments
  KVM: X86: Fix read out-of-bounds vulnerability in kvm pio emulation
  KVM: x86: Fix potential preemption when get the current kvmclock timestamp
  KVM: Silence underflow warning in avic_get_physical_id_entry()
  KVM: arm/arm64: Hold slots_lock when unregistering kvm io bus devices
  KVM: arm/arm64: Fix bug when registering redist iodevs
  KVM: x86: lower default for halt_poll_ns
  kvm: arm/arm64: Fix use after free of stage2 page table
  kvm: arm/arm64: Force reading uncached stage2 PGD
  KVM: nVMX: fix EPT permissions as reported in exit qualification
  KVM: VMX: Don't enable EPT A/D feature if EPT feature is disabled
  KVM: x86: Fix load damaged SSEx MXCSR register
  kvm: nVMX: off by one in vmx_write_pml_buffer()
  KVM: arm: rename pm_fake handler to trap_raz_wi
  KVM: arm: plug potential guest hardware debug leakage
  kvm: arm/arm64: Fix race in resetting stage2 PGD
  KVM: arm/arm64: vgic-v3: Use PREbits to infer the number of ICH_APxRn_EL2 registers
  KVM: arm/arm64: vgic-v3: Do not use Active+Pending state for a HW interrupt
  ...

…inux/kernel/git/xen/tip

Pull xen fixes from Juergen Gross:
 "Some fixes for the new Xen 9pfs frontend and some minor cleanups"

* tag 'for-linus-4.12b-rc2-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip:
  xen: make xen_flush_tlb_all() static
  xen: cleanup pvh leftovers from pv-only sources
  xen/9pfs: p9_trans_xen_init and p9_trans_xen_exit can be static
  xen/9pfs: fix return value check in xen_9pfs_front_probe()