workqueue: Clean code in alloc_and_link_pwqs()

wq->flags would not change, so it's not necessary to check if WQ_BH
is set in loop for_each_possible_cpu(), move define and set of pools
out of loop to simpliy the code.

Signed-off-by: Wenchao Hao <haowenchao22@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>