block: remove WARN_ON() from bd_link_disk_holder

Since commit 83cbce95("block: add error handling for device_add_disk /
add_disk"), bdev->bd_holder_dir can not be empty now, so remove WARN_ON()
from bd_link_disk_holder.
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220623074100.2251301-1-linan122@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/holder.c

@@ -79,10 +79,6 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
 
 	WARN_ON_ONCE(!bdev->bd_holder);
 
-	/* FIXME: remove the following once add_disk() handles errors */
-	if (WARN_ON(!bdev->bd_holder_dir))
-		goto out_unlock;
-
 	holder = bd_find_holder_disk(bdev, disk);
 	if (holder) {
 		holder->refcnt++;