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>

kernel/workqueue_internal.h

@@ -37,7 +37,7 @@ struct worker {
 	/* 64 bytes boundary on 64bit, 32 on 32bit */
 
 	struct task_struct	*task;		/* I: worker task */
-	struct worker_pool	*pool;		/* I: the associated pool */
+	struct worker_pool	*pool;		/* A: the associated pool */
 						/* L: for rescuers */
 	struct list_head	node;		/* A: anchored at pool->workers */
 						/* A: runs through worker->node */