fs/super.c: don't drop ->s_user_ns until we free struct super_block itself

Avoids fun races in RCU pathwalk...  Same goes for freeing LSM shite
hanging off super_block's arse.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/super.c

@@ -274,9 +274,10 @@ static void destroy_super_work(struct work_struct *work)
 {
 	struct super_block *s = container_of(work, struct super_block,
 							destroy_work);
-	int i;
-
-	for (i = 0; i < SB_FREEZE_LEVELS; i++)
+	security_sb_free(s);
+	put_user_ns(s->s_user_ns);
+	kfree(s->s_subtype);
+	for (int i = 0; i < SB_FREEZE_LEVELS; i++)
 		percpu_free_rwsem(&s->s_writers.rw_sem[i]);
 	kfree(s);
 }
@@ -296,9 +297,6 @@ static void destroy_unused_super(struct super_block *s)
 	super_unlock_excl(s);
 	list_lru_destroy(&s->s_dentry_lru);
 	list_lru_destroy(&s->s_inode_lru);
-	security_sb_free(s);
-	put_user_ns(s->s_user_ns);
-	kfree(s->s_subtype);
 	shrinker_free(s->s_shrink);
 	/* no delays needed */
 	destroy_super_work(&s->destroy_work);
@@ -409,9 +407,6 @@ static void __put_super(struct super_block *s)
 		WARN_ON(s->s_dentry_lru.node);
 		WARN_ON(s->s_inode_lru.node);
 		WARN_ON(!list_empty(&s->s_mounts));
-		security_sb_free(s);
-		put_user_ns(s->s_user_ns);
-		kfree(s->s_subtype);
 		call_rcu(&s->rcu, destroy_super_rcu);
 	}
 }