dm: only initialize the request_queue once

Commit bfebd1cdb4 ("dm: add full blk-mq support to request-based DM")
didn't properly account for the need to short-circuit re-initializing
DM's blk-mq request_queue if it was already initialized.

Otherwise, reloading a blk-mq request-based DM table (either manually
or via multipathd) resulted in errors, see:
https://www.redhat.com/archives/dm-devel/2015-April/msg00132.html

Fix is to only initialize the request_queue on the initial table load
(when the mapped_device type is assigned).

This is better than having dm_init_request_based_blk_mq_queue() return
early if the queue was already initialized because it elevates the
constraint to a more meaningful location in DM core. As such the
pre-existing early return in dm_init_request_based_queue() can now be
removed.

Fixes: bfebd1cdb4 ("dm: add full blk-mq support to request-based DM")
Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-ioctl.c

···
 		goto err_unlock_md_type;
 	}
 
-	if (dm_get_md_type(md) == DM_TYPE_NONE)
+	if (dm_get_md_type(md) == DM_TYPE_NONE) {
 		/* Initial table load: acquire type of table. */
 		dm_set_md_type(md, dm_table_get_type(t));
-	else if (dm_get_md_type(md) != dm_table_get_type(t)) {
+
+		/* setup md->queue to reflect md's type (may block) */
+		r = dm_setup_md_queue(md);
+		if (r) {
+			DMWARN("unable to set up device queue for new table.");
+			goto err_unlock_md_type;
+		}
+	} else if (dm_get_md_type(md) != dm_table_get_type(t)) {
 		DMWARN("can't change device type after initial table load.");
 		r = -EINVAL;
 		goto err_unlock_md_type;
 	}
 
-	/* setup md->queue to reflect md's type (may block) */
-	r = dm_setup_md_queue(md);
-	if (r) {
-		DMWARN("unable to set up device queue for new table.");
-		goto err_unlock_md_type;
-	}
 	dm_unlock_md_type(md);
 
 	/* stage inactive table */

drivers/md/dm.c

···
 {
 	struct request_queue *q = NULL;
 
-	if (md->queue->elevator)
-		return 0;
-
 	/* Fully initialize the queue */
 	q = blk_init_allocated_queue(md->queue, dm_request_fn, NULL);
 	if (!q)