dm bufio: remove use of deprecated create_singlethread_workqueue()

The workqueue "dm_bufio_wq" queues a single work item &dm_bufio_work so
it doesn't require execution ordering.  Hence, alloc_workqueue() has
been used to replace the deprecated create_singlethread_workqueue().

The WQ_MEM_RECLAIM flag has been set since DM requires forward progress
under memory pressure.

Since there are fixed number of work items, explicit concurrency
limit is unnecessary here.
Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-bufio.c

@@ -1879,7 +1879,7 @@ static int __init dm_bufio_init(void)
 	__cache_size_refresh();
 	mutex_unlock(&dm_bufio_clients_lock);
 
-	dm_bufio_wq = create_singlethread_workqueue("dm_bufio_cache");
+	dm_bufio_wq = alloc_workqueue("dm_bufio_cache", WQ_MEM_RECLAIM, 0);
 	if (!dm_bufio_wq)
 		return -ENOMEM;