workqueue: add find_worker_executing_work() and track current_cwq

Now that all the workers are tracked by gcwq, we can find which worker
is executing a work from gcwq.  Implement find_worker_executing_work()
and make worker track its current_cwq so that we can find things the
other way around.  This will be used to implement non-reentrant wqs.

Signed-off-by: Tejun Heo <tj@kernel.org>