workqueue: wake regular worker if need_more_worker() when rescuer leave the pool

We don't need to wake up regular worker when nr_running==1,
so need_more_worker() is sufficient here.

And need_more_worker() gives us better readability due to the name of
"keep_working()" implies the rescuer should keep working now but
the rescuer is actually leaving.

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