workqueue: Remove the mb() pair between wq_worker_sleeping() and insert_work()

In wq_worker_sleeping(), the access to worklist is protected by the
pool->lock, so the memory barrier is unneeded.

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