workqueue: make rescuer_thread() empty wq->maydays list before exiting

After a @pwq is scheduled for emergency execution, other workers may
consume the affectd work items before the rescuer gets to them.  This
means that a workqueue many have pwqs queued on @wq->maydays list
while not having any work item pending or in-flight.  If
destroy_workqueue() executes in such condition, the rescuer may exit
without emptying @wq->maydays.

This currently doesn't cause any actual harm.  destroy_workqueue() can
safely destroy all the involved data structures whether @wq->maydays
is populated or not as nobody access the list once the rescuer exits.

However, this is nasty and makes future development difficult.  Let's
update rescuer_thread() so that it empties @wq->maydays after seeing
should_stop to guarantee that the list is empty on rescuer exit.

tj: Updated comment and patch description.

Signed-off-by: Lai Jiangshan <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
Cc: [email protected] # v3.10+

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