workqueue: Change the comments of the synchronization about the idle_list

The access to idle_list in wq_worker_sleeping() is changed to be
protected by pool->lock, so the comments above idle_list can be changed
to "L:" which is the meaning of "access with pool->lock held".

And the outdated comments in wq_worker_sleeping() is removed since
the function is not called with rq lock held any more, idle_list is
dereferenced with pool lock now.

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