workqueue: Link pwq's into wq->pwqs from oldest to newest

Add a new pwq into the tail of wq->pwqs so that pwq iteration will
start from the oldest pwq to the newest. This ordering will facilitate
the inclusion of ordered workqueues in a wq_unbound_cpumask update.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>