blk-mq-sched: Enable merging discard bio into request

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-sched.c

@@ -259,6 +259,8 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
 		if (!*merged_request)
 			elv_merged_request(q, rq, ELEVATOR_FRONT_MERGE);
 		return true;
+	case ELEVATOR_DISCARD_MERGE:
+		return bio_attempt_discard_merge(q, rq, bio);
 	default:
 		return false;
 	}