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>