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.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>