(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>