workqueue: simplify CPU hotplug code

With trustee gone, CPU hotplug code can be simplified.

* gcwq_claim/release_management() now grab and release gcwq lock too
respectively and gained _and_lock and _and_unlock postfixes.

* All CPU hotplug logic was implemented in workqueue_cpu_callback()
which was called by workqueue_cpu_up/down_callback() for the correct
priority.  This was because up and down paths shared a lot of logic,
which is no longer true.  Remove workqueue_cpu_callback() and move
all hotplug logic into the two actual callbacks.

This patch doesn't make any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>