fsnotify: do not handle events on a shutting down filesystem

Protect against use after free when filesystem calls fsnotify_sb_error()
after calling fsnotify_sb_delete().

Reported-by: syzbot+5e3f9b2a67b45f16d4e6@syzkaller.appspotmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

include/linux/fsnotify.h

@@ -23,6 +23,9 @@ static inline bool fsnotify_sb_has_priority_watchers(struct super_block *sb,
 {
 	struct fsnotify_sb_info *sbinfo = fsnotify_sb_info(sb);
 
+	if (unlikely(!(sb->s_flags & SB_ACTIVE)))
+		return 0;
+
 	/* Were any marks ever added to any object on this sb? */
 	if (!sbinfo)
 		return false;
@@ -453,6 +456,9 @@ static inline int fsnotify_sb_error(struct super_block *sb, struct inode *inode,
 		.sb = sb,
 	};
 
+	if (!fsnotify_sb_has_watchers(sb))
+		return 0;
+
 	return fsnotify(FS_ERROR, &report, FSNOTIFY_EVENT_ERROR,
 			NULL, NULL, NULL, 0);
 }