(namespace) ipc: Initialize ipc_namespace->user_ns early.

BugLink: http://bugs.launchpad.net/bugs/1588056

Allow the ipc namespace initialization code to depend on ns->user_ns
being set during initialization.

In particular this allows mq_init_ns to use ns->user_ns for permission
checks and initializating s_user_ns while the the mq filesystem is
being mounted.
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Suggested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
(cherry picked from linux-next commit b236017a)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

ipc/namespace.c

@@ -34,8 +34,11 @@ static struct ipc_namespace *create_ipc_ns(struct user_namespace *user_ns,
 	ns->ns.ops = &ipcns_operations;
 
 	atomic_set(&ns->count, 1);
+	ns->user_ns = get_user_ns(user_ns);
+
 	err = mq_init_ns(ns);
 	if (err) {
+		put_user_ns(ns->user_ns);
 		ns_free_inum(&ns->ns);
 		kfree(ns);
 		return ERR_PTR(err);
@@ -46,8 +49,6 @@ static struct ipc_namespace *create_ipc_ns(struct user_namespace *user_ns,
 	msg_init_ns(ns);
 	shm_init_ns(ns);
 
-	ns->user_ns = get_user_ns(user_ns);
-
 	return ns;
 }