revert kthread_work: wake up worker only when the worker is idle

If the worker is already executing a work item when another is queued,
we can safely skip wakeup without worrying about stalling queue thus
avoiding waking up the busy worker spuriously.  Spurious wakeups
should be fine but still isn't nice and avoiding it is trivial here.

tj: Updated description.

Change-Id: I760735be651e88b82be56db454ebb06adea1380c
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
Signed-off-by: engstk <eng.stk@sapo.pt> (reverted from commit c921a43afb764c35e7f0743cd1023e87a8ef6913)