fsnotify: use an explicit flag to indicate fsnotify_destroy_mark has been called

Currently fsnotify check is mark->group is NULL to decide if
fsnotify_destroy_mark() has already been called or not.  With the upcoming
rcu work it is a heck of a lot easier to use an explicit flag than worry
about group being set to NULL.

Signed-off-by: Eric Paris <eparis@redhat.com>
diff --git a/fs/notify/inode_mark.c b/fs/notify/inode_mark.c
index 455cb41..37b460f 100644
--- a/fs/notify/inode_mark.c
+++ b/fs/notify/inode_mark.c
@@ -187,7 +187,7 @@
 	struct hlist_node *node, *last = NULL;
 	int ret = 0;
 
-	mark->flags = FSNOTIFY_MARK_FLAG_INODE;
+	mark->flags |= FSNOTIFY_MARK_FLAG_INODE;
 
 	assert_spin_locked(&mark->lock);
 	assert_spin_locked(&group->mark_lock);