dm: convert workqueues to alloc_ordered

Convert all create[_singlethread]_work() users to the new
alloc[_ordered]_workqueue().  This conversion is mechanical and
doesn't introduce any behavior change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-snap-persistent.c

@@ -818,7 +818,7 @@ static int persistent_ctr(struct dm_exception_store *store,
 	atomic_set(&ps->pending_count, 0);
 	ps->callbacks = NULL;
 
-	ps->metadata_wq = create_singlethread_workqueue("ksnaphd");
+	ps->metadata_wq = alloc_ordered_workqueue("ksnaphd", WQ_MEM_RECLAIM);
 	if (!ps->metadata_wq) {
 		kfree(ps);
 		DMERR("couldn't start header metadata update thread");