xfs: convert to alloc_workqueue()

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>
diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index ac1c7e8..f83a4c8 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -2022,11 +2022,12 @@
 	if (!xfslogd_workqueue)
 		goto out_free_buf_zone;
 
-	xfsdatad_workqueue = create_workqueue("xfsdatad");
+	xfsdatad_workqueue = alloc_workqueue("xfsdatad", WQ_MEM_RECLAIM, 1);
 	if (!xfsdatad_workqueue)
 		goto out_destroy_xfslogd_workqueue;
 
-	xfsconvertd_workqueue = create_workqueue("xfsconvertd");
+	xfsconvertd_workqueue = alloc_workqueue("xfsconvertd",
+						WQ_MEM_RECLAIM, 1);
 	if (!xfsconvertd_workqueue)
 		goto out_destroy_xfsdatad_workqueue;