workqueue: remove the empty check in too_many_workers()

The commit ea1abd6197 ("workqueue: reimplement idle worker rebinding")
used a trick which simply removes all to-be-bound idle workers from the
idle list and lets them add themselves back after completing rebinding.

And this trick caused the @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.

The commit/trick was complicated due to it just tried to simplify an even
more complicated problem (workers had to rebind itself). But the commit
a9ab775bca ("workqueue: directly restore CPU affinity of workers
from CPU_ONLINE") fixed all these problems and the mentioned trick was
useless and is gone.

So, now the @worker_pool->nr_idle is exactly the actual number of workers
on @worker_pool->idle_list. too_many_workers() should recover as it was
before the trick. So we remove the empty check.

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

kernel/workqueue.c

@@ -758,13 +758,6 @@ static bool too_many_workers(struct worker_pool *pool)
 	int nr_idle = pool->nr_idle + managing; /* manager is considered idle */
 	int nr_busy = pool->nr_workers - nr_idle;
 
-	/*
-	 * nr_idle and idle_list may disagree if idle rebinding is in
-	 * progress.  Never return %true if idle_list is empty.
-	 */
-	if (list_empty(&pool->idle_list))
-		return false;
-
 	return nr_idle > 2 && (nr_idle - 2) * MAX_IDLE_WORKERS_RATIO >= nr_busy;
 }