workqueue: Rename wq_update_pod() to unbound_wq_update_pwq()

What wq_update_pod() does is just to update the pwq of the specific
cpu.  Rename it and update the comments.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Tejun Heo <tj@kernel.org>