fsnotify: convert fsnotify_mark.refcnt from atomic_t to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable fsnotify_mark.refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>

include/linux/fsnotify_backend.h

@@ -242,7 +242,7 @@ struct fsnotify_mark {
 	__u32 mask;
 	/* We hold one for presence in g_list. Also one ref for each 'thing'
 	 * in kernel that found and may be using this mark. */
-	atomic_t refcnt;
+	refcount_t refcnt;
 	/* Group this mark is for. Set on mark creation, stable until last ref
 	 * is dropped */
 	struct fsnotify_group *group;