blk-mq-sched: (un)register elevator when (un)registering queue

I noticed that when booting with a default blk-mq I/O scheduler, the
/sys/block/*/queue/iosched directory was missing. However, switching
after boot did create the directory. This is because we skip the initial
elevator register/unregister when we don't have a ->request_fn(), but we
should still do it for the ->mq_ops case.
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-sysfs.c

@@ -903,9 +903,7 @@ int blk_register_queue(struct gendisk *disk)
 
 	blk_wb_init(q);
 
-	if (!q->request_fn)
-		return 0;
-
+	if (q->request_fn || (q->mq_ops && q->elevator)) {
 		ret = elv_register_queue(q);
 		if (ret) {
 			kobject_uevent(&q->kobj, KOBJ_REMOVE);
@@ -914,6 +912,7 @@ int blk_register_queue(struct gendisk *disk)
 			kobject_put(&dev->kobj);
 			return ret;
 		}
+	}
 
 	return 0;
 }
@@ -928,7 +927,7 @@ void blk_unregister_queue(struct gendisk *disk)
 	if (q->mq_ops)
 		blk_mq_unregister_dev(disk_to_dev(disk), q);
 
-	if (q->request_fn)
+	if (q->request_fn || (q->mq_ops && q->elevator))
 		elv_unregister_queue(q);
 
 	kobject_uevent(&q->kobj, KOBJ_REMOVE);