blk-mq: Only register debugfs attributes for blk-mq queues

The code in blk-mq-debugfs.c assumes that it is working on a blk-mq
queue and is not intended to work on a blk-sq queue. Hence only
register blk-mq debugfs attributes for blk-mq queues.

Fixes: commit 9c1051aacde8 ("blk-mq: untangle debugfs and sysfs")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-sysfs.c

@@ -887,10 +887,10 @@ int blk_register_queue(struct gendisk *disk)
 		goto unlock;
 	}
 
-	if (q->mq_ops)
+	if (q->mq_ops) {
 		__blk_mq_register_dev(dev, q);
-
-	blk_mq_debugfs_register(q);
+		blk_mq_debugfs_register(q);
+	}
 
 	kobject_uevent(&q->kobj, KOBJ_ADD);