shpchp: update workqueue usage

* Rename shpchp_wq to shpchp_ordered_wq and add non-ordered shpchp_wq
  which is used instead of the system workqueue.  This is to remove
  the use of flush_scheduled_work() which is deprecated and scheduled
  for removal.

* With cmwq in place, there's no point in creating workqueues lazily.
  Create both shpchp_wq and shpchp_ordered_wq upfront.

* Include workqueue.h from shpchp.h.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>