block: remove the extra kobject reference in bd_link_disk_holder

Since commit 0d02129e ("block: merge struct block_device and struct
hd_struct") there is no way for the bdev to go away as long as there is
a holder, so remove the extra references.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20210804094147.459763-3-hch@lst.de


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