kernel/workqueue: Let rescuers follow unbound wq cpumask changes

kernel/workqueue: Let rescuers follow unbound wq cpumask changes

When workqueue cpumask changes are committed the associated rescuer (if
one exists) affinity is not touched and this might be a problem down the
line for isolated setups.

Make sure rescuers affinity is updated every time a workqueue cpumask
changes, so that rescuers can't break isolation.

 [longman: set_cpus_allowed_ptr() will block until the designated task
  is enqueued on an allowed CPU, no wake_up_process() needed. Also use
  the unbound_effective_cpumask() helper as suggested by Tejun.]

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