block: don't handle bio based drivers in blk_revalidate_disk_zones

bio based drivers only need to update q->nr_zones.  Do that manually
instead of overloading blk_revalidate_disk_zones to keep that function
simpler for the next round of changes that will rely even more on the
request based functionality.

Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -1576,11 +1576,17 @@ static int null_gendisk_register(struct nullb *nullb)
 	disk->queue		= nullb->q;
 	strncpy(disk->disk_name, nullb->disk_name, DISK_NAME_LEN);
 
+#ifdef CONFIG_BLK_DEV_ZONED
 	if (nullb->dev->zoned) {
-		ret = blk_revalidate_disk_zones(disk);
-		if (ret)
-			return ret;
+		if (queue_is_mq(nullb->q)) {
+			ret = blk_revalidate_disk_zones(disk);
+			if (ret)
+				return ret;
+		} else {
+			nullb->q->nr_zones = blkdev_nr_zones(disk);
+		}
 	}
+#endif
 
 	add_disk(disk);
 	return 0;