workqueue: Remember whether a work item was on a BH workqueue

Add an off-queue flag, WORK_OFFQ_BH, that indicates whether the last
workqueue the work item was on was a BH one. This will be used to test
whether a work item is BH in cancel_sync path to implement atomic
cancel_sync'ing for BH work items.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>