sched: don't forget to unlock uids_mutex on error paths

The commit

 commit 5cb350baf580017da38199625b7365b1763d7180
 Author: Dhaval Giani <dhaval@linux.vnet.ibm.com>
 Date:   Mon Oct 15 17:00:14 2007 +0200

    sched: group scheduling, sysfs tunables

introduced the uids_mutex and the helpers to lock/unlock it.
Unfortunately, the error paths of alloc_uid() were not patched
to unlock it.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/user.c

@@ -337,8 +337,11 @@ struct user_struct * alloc_uid(struct user_namespace *ns, uid_t uid)
 		struct user_struct *new;
 
 		new = kmem_cache_alloc(uid_cachep, GFP_KERNEL);
-		if (!new)
+		if (!new) {
+			uids_mutex_unlock();
 			return NULL;
+		}
+
 		new->uid = uid;
 		atomic_set(&new->__count, 1);
 		atomic_set(&new->processes, 0);
@@ -355,6 +358,7 @@ struct user_struct * alloc_uid(struct user_namespace *ns, uid_t uid)
 
 		if (alloc_uid_keyring(new, current) < 0) {
 			kmem_cache_free(uid_cachep, new);
+			uids_mutex_unlock();
 			return NULL;
 		}
 
@@ -362,6 +366,7 @@ struct user_struct * alloc_uid(struct user_namespace *ns, uid_t uid)
 			key_put(new->uid_keyring);
 			key_put(new->session_keyring);
 			kmem_cache_free(uid_cachep, new);
+			uids_mutex_unlock();
 			return NULL;
 		}