workqueue: reimplement idle worker rebinding

Currently rebind_workers() uses rebinds idle workers synchronously
before proceeding to requesting busy workers to rebind.  This is
necessary because all workers on @worker_pool->idle_list must be bound
before concurrency management local wake-ups from the busy workers
take place.

Unfortunately, the synchronous idle rebinding is quite complicated.
This patch reimplements idle rebinding to simplify the code path.

Rather than trying to make all idle workers bound before rebinding
busy workers, we simply remove all to-be-bound idle workers from the
idle list and let them add themselves back after completing rebinding
(successful or not).

As only workers which finished rebinding can on on the idle worker
list, the idle worker list is guaranteed to have only bound workers
unless CPU went down again and local wake-ups are safe.

After the change, @worker_pool->nr_idle may deviate than the actual
number of idle workers on @worker_pool->idle_list.  More specifically,
nr_idle may be non-zero while ->idle_list is empty.  All users of
->nr_idle and ->idle_list are audited.  The only affected one is
too_many_workers() which is updated to check %false if ->idle_list is
empty regardless of ->nr_idle.

After this patch, rebind_workers() no longer performs the nasty
idle-rebind retries which require temporary release of gcwq->lock, and
both unbinding and rebinding are atomic w.r.t. global_cwq->lock.

worker->idle_rebind and global_cwq->rebind_hold are now unnecessary
and removed along with the definition of struct idle_rebind.

Changed from V1:
	1) remove unlikely from too_many_workers(), ->idle_list can be empty
	   anytime, even before this patch, no reason to use unlikely.
	2) fix a small rebasing mistake.
	   (which is from rebasing the orignal fixing patch to for-next)
	3) add a lot of comments.
	4) clear WORKER_REBIND unconditionaly in idle_worker_rebind()

tj: Updated comments and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>