type refcount_t added to linux kernel 4.11 conflicts with zfs-defined type

[ofaaland]

System information

Type	Version/Name
Distribution Name
Distribution Version
Linux Kernel	4.11
Architecture
ZFS Version	master
SPL Version	master

Describe the problem you're observing

Both kernel.org builders are failing to build every zfs patch. ZFS defines refcount_t but the kernel has already defined that identifier.

Describe how to reproduce the problem

Build master against torvalds/linux after Feb 20 2017 when
commit f405df5 refcount_t: Introduce a special purpose refcount type
was merged with commit 42e1b14

Include any warning/errors/backtraces from the system logs

 } refcount_t;
   ^
In file included from ./include/linux/kref.h:19:0,
                 from ./include/linux/kobject.h:24,
                 from ./include/linux/module.h:17,
                 from ./include/spl/sys/sysmacros.h:28,
                 from ./include/spl/sys/types.h:29,
                 from ./include/zfs/sys/zfs_context.h:34,
                 from fs/zfs/zcommon/zfs_deleg.c:28:
./include/linux/refcount.h:55:3: note: previous declaration of ‘refcount_t’ was here
 } refcount_t;
   ^

[ofaaland]

linux kernel defines refcount_t in new file include/linux/refcount.h:

typedef struct refcount_struct {
       atomic_t refs;
} refcount_t;

guarded by _LINUX_REFCOUNT_H