fsnotify: don't BUG in fsnotify_destroy_mark()

Removing the parent of a watched file results in "kernel BUG at
fs/notify/mark.c:139".

To reproduce

  add "-w /tmp/audit/dir/watched_file" to audit.rules
  rm -rf /tmp/audit/dir

This is caused by fsnotify_destroy_mark() being called without an
extra reference taken by the caller.

Reported by Francesco Cosoleto here:

  https://bugzilla.novell.com/show_bug.cgi?id=689860

Fix by removing the BUG_ON and adding a comment about not accessing mark after
the iput.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/notify/mark.c

@@ -135,9 +135,6 @@ void fsnotify_destroy_mark(struct fsnotify_mark *mark)
 
 	mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 
-	/* 1 from caller and 1 for being on i_list/g_list */
-	BUG_ON(atomic_read(&mark->refcnt) < 2);
-
 	spin_lock(&group->mark_lock);
 
 	if (mark->flags & FSNOTIFY_MARK_FLAG_INODE) {
@@ -181,6 +178,11 @@ void fsnotify_destroy_mark(struct fsnotify_mark *mark)
 	if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
 		iput(inode);
 
+	/*
+	 * We don't necessarily have a ref on mark from caller so the above iput
+	 * may have already destroyed it.  Don't touch from now on.
+	 */
+
 	/*
 	 * it's possible that this group tried to destroy itself, but this
 	 * this mark was simultaneously being freed by inode.  If that's the