BugLink: http://bugs.launchpad.net/bugs/1566505

This ensures that quotas that aren't valid on disk never make it
into the hash table. inodes with ids which don't map into
s_user_ns will now have no quota enforcement.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>