fuse: convert fuse_req.count from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/fuse_i.h

@@ -307,7 +307,7 @@ struct fuse_req {
 	struct list_head intr_entry;
 
 	/** refcount */
-	atomic_t count;
+	refcount_t count;
 
 	/** Unique ID for the interrupt request */
 	u64 intr_unique;