UBUNTU: SAUCE: quota: Require that qids passed to dqget() be valid and map into s_user_ns

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>