workqueue: post global_cwq removal cleanups

Remove remaining references to gcwq.

* __next_gcwq_cpu() steals __next_wq_cpu() name.  The original
  __next_wq_cpu() became __next_cwq_cpu().

* s/for_each_gcwq_cpu/for_each_wq_cpu/
  s/for_each_online_gcwq_cpu/for_each_online_wq_cpu/

* s/gcwq_mayday_timeout/pool_mayday_timeout/

* s/gcwq_unbind_fn/wq_unbind_fn/

* Drop references to gcwq in comments.

This patch doesn't introduce any functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>