workqueue: Make worker_attach/detach_pool() update worker->pool
For historical reasons, the worker attach/detach functions don't
currently manage worker->pool and the callers are manually and
inconsistently updating it.
This patch moves worker->pool updates into the worker attach/detach
functions. This makes worker->pool consistent and clearly defines how
worker->pool updates are synchronized.
This will help later workqueue visibility improvements by allowing
safe access to workqueue information from worker->task.
Signed-off-by: Tejun Heo <tj@kernel.org>