workqueue: factor out initial worker creation into create_and_start_worker()

workqueue: factor out initial worker creation into create_and_start_worker()

get_unbound_pool(), workqueue_cpu_up_callback() and init_workqueues()
have similar code pieces to create and start the initial worker factor
those out into create_and_start_worker().

This patch doesn't introduce any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>