dm: distinquish old .request_fn (dm-old) vs dm-mq request-based DM

Rename various methods to have either a "dm_old" or "dm_mq" prefix.
Improve code comments to assist with understanding the duality of code
that handles both "dm_old" and "dm_mq" cases.

It is no much easier to quickly look at the code and _know_ that a given
method is either 1) "dm_old" only 2) "dm_mq" only 3) common to both.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-mpath.c

@@ -426,7 +426,11 @@ static int __multipath_map(struct dm_target *ti, struct request *clone,
 		clone->rq_disk = bdev->bd_disk;
 		clone->cmd_flags |= REQ_FAILFAST_TRANSPORT;
 	} else {
-		/* blk-mq request-based interface */
+		/*
+		 * blk-mq request-based interface; used by both:
+		 * .request_fn stacked on blk-mq path(s) and
+		 * blk-mq stacked on blk-mq path(s).
+		 */
 		*__clone = blk_get_request(bdev_get_queue(bdev),
 					   rq_data_dir(rq), GFP_ATOMIC);
 		if (IS_ERR(*__clone)) {