KVM: Put mm immediately after async #PF worker completes remote gup()

Put the async #PF worker's reference to the VM's address space as soon as
the worker is done with the mm.  This will allow deferring getting a
reference to the worker itself without having to track whether or not
getting a reference succeeded.

Note, if the vCPU is still alive, there is no danger of the worker getting
stuck with tearing down the host page tables, as userspace also holds a
reference (obviously), i.e. there is no risk of delaying the page-present
notification due to triggering the slow path in mmput().

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240110011533.503302-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

virt/kvm/async_pf.c

@@ -64,6 +64,7 @@ static void async_pf_execute(struct work_struct *work)
 	get_user_pages_remote(mm, addr, 1, FOLL_WRITE, NULL, &locked);
 	if (locked)
 		mmap_read_unlock(mm);
+	mmput(mm);
 
 	if (IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC))
 		kvm_arch_async_page_present(vcpu, apf);
@@ -85,8 +86,6 @@ static void async_pf_execute(struct work_struct *work)
 	trace_kvm_async_pf_completed(addr, cr2_or_gpa);
 
 	__kvm_vcpu_wake_up(vcpu);
-
-	mmput(mm);
 }
 
 static void kvm_flush_and_free_async_pf_work(struct kvm_async_pf *work)