fix freeing user_struct in user cache

When racing on adding into user cache, the new allocated from mm slab
is freed without putting user namespace.

Since the user namespace is already operated by getting, putting has
to be issued.
Signed-off-by: NHillf Danton <dhillf@gmail.com>
Acked-by: NSerge Hallyn <serge@hallyn.com>
Cc: stable@kernel.org
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/user.c

@@ -158,6 +158,7 @@ struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
 		spin_lock_irq(&uidhash_lock);
 		up = uid_hash_find(uid, hashent);
 		if (up) {
+			put_user_ns(ns);
 			key_put(new->uid_keyring);
 			key_put(new->session_keyring);
 			kmem_cache_free(uid_cachep, new);