Introduce for_each_pwq() which iterates all pool_workqueues of a
workqueue using the recently added workqueue->pwqs list and replace
for_each_pwq_cpu() usages with it.

This is primarily to remove the single unbound CPU assumption from pwq
iteration for the scheduled unbound pools with custom attributes
support which would introduce multiple unbound pwqs per workqueue;
however, it also simplifies iterator users.

Note that pwq->pool initialization is moved to alloc_and_link_pwqs()
as that now is the only place which is explicitly handling the two pwq
types.

This patch doesn't introduce any visible behavior changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>