workqueue: implement WQ_NON_REENTRANT

With gcwq managing all the workers and work->data pointing to the last
gcwq it was on, non-reentrance can be easily implemented by checking
whether the work is still running on the previous gcwq on queueing.
Implement it.

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

diff --cc include/linux/workqueue.h
Simple merge

diff --cc kernel/workqueue.c
Simple merge