workqueue: replace for_each_worker_pool() with for_each_std_worker_pool()

for_each_std_worker_pool() takes @cpu instead of @gcwq.

This is part of an effort to remove global_cwq and make worker_pool
the top level abstraction, which in turn will help implementing worker
pools with user-specified attributes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>