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: NTejun Heo <tj@kernel.org>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>