This reverts commit aa24163b2ee5c92120e32e99b5a93143a0f4258e.
This and the following commit led to another circular locking scenario
and the scenario which is fixed by this commit no longer exists after
e8b3f8db7aad ("workqueue/hotplug: simplify workqueue_offline_cpu()")
which removes work item flushing from hotplug path.
Revert it for now.
Signed-off-by: Tejun Heo <tj@kernel.org>