dnotify: Handle errors from fsnotify_add_mark_locked() in fcntl_dirnotify()

fsnotify_add_mark_locked() can fail but we do not check its return
value. This didn't matter before commit 9dd813c15b "fsnotify: Move
mark list head from object into dedicated structure" as none of possible
failures could happen for dnotify but after that commit -ENOMEM can be
returned. Handle this error properly in fcntl_dirnotify() as
otherwise we just hit BUG_ON(dn_mark->dn) in dnotify_free_mark().

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reported-by: syzkaller
Fixes: 9dd813c15b
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/dnotify/dnotify.c

@@ -319,7 +319,11 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg)
 		dn_mark = container_of(fsn_mark, struct dnotify_mark, fsn_mark);
 		spin_lock(&fsn_mark->lock);
 	} else {
-		fsnotify_add_mark_locked(new_fsn_mark, inode, NULL, 0);
+		error = fsnotify_add_mark_locked(new_fsn_mark, inode, NULL, 0);
+		if (error) {
+			mutex_unlock(&dnotify_group->mark_mutex);
+			goto out_err;
+		}
 		spin_lock(&new_fsn_mark->lock);
 		fsn_mark = new_fsn_mark;
 		dn_mark = new_dn_mark;
@@ -345,6 +349,7 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg)
 		 */
 		if (dn_mark == new_dn_mark)
 			destroy = 1;
+		error = 0;
 		goto out;
 	}