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

Sync up with changes from Eric Biederman when merging s_user_ns
support upstream. Partial backport of
6e4eab57 from linux-next.
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>