Convert from create[_singlethread]_workqueue() to alloc_workqueue().

* xfsdatad_workqueue and xfsconvertd_workqueue are identity converted.
  Using higher concurrency limit might be useful but given the
  complexity of workqueue usage in xfs, proceeding cautiously seems
  better.

* xfs_mru_reap_wq is converted to non-ordered workqueue with max
  concurrency of 1 as the work items don't require any specific
  ordering and already have proper synchronization.  It seems it was
  singlethreaded to save worker threads, which is no longer a concern.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Alex Elder <aelder@sgi.com>
Cc: xfs-masters@oss.sgi.com
Cc: Christoph Hellwig <hch@infradead.org>