workqueue: Changes for v6.19

- Rescuer affinity management: Affinity now updated only when detached using
  wq_unbound_cpumask consistently. DISASSOCIATED workers also follow unbound
  cpumask changes to avoid breaking CPU isolation.

- Rescuer cleanups preparing for fetching work items one by one from pool list:
  Work assignment factored out, optimized to skip pwqs no longer needing
  rescue, and shutdown logic simplified.

- Unused assert_rcu_or_wq_mutex_or_pool_mutex() removed.

workqueue: Don't rely on wq->rescuer to stop rescuer

The commit1 def98c84b6cd ("workqueue: Fix spurious sanity check failures
in destroy_workqueue()") tries to fix spurious sanity check failures by
stopping send_mayday() via setting wq->rescuer to NULL.

But it fails to stop the pwq->mayday_node requeuing in the rescuer, and
the commit2 e66b39af00f4 ("workqueue: Fix pwq ref leak in
rescuer_thread()") fixes it by checking wq->rescuer which is the result
of commit1.

Both commits together really fix spurious sanity check failures caused
by the rescuer, but they both use a convoluted method by relying on
wq->rescuer state rather than the real count of work items.

Actually __WQ_DESTROYING and drain_workqueue() together already stop
send_mayday() by draining all the work items and ensuring no new work
item requeuing.

And the more proper fix to stop the pwq->mayday_node requeuing in the
rescuer is from commit3 4f3f4cf388f8 ("workqueue: avoid unneeded
requeuing the pwq in rescuer thread") and renders the checking of
wq->rescuer in commit2 unnecessary.

So __WQ_DESTROYING, drain_workqueue() and commit3 together fix spurious
sanity check failures introduced by the rescuer.

Just remove the convoluted code of using wq->rescuer.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Tejun Heo <tj@kernel.org>