block: add back the bd_holder_dir reference in bd_link_disk_holder

This essentially reverts "block: remove the extra kobject reference in
bd_link_disk_holder".  That commit dropped the extra reference because
the condition in the comment can't be true.  But it turns out that
comment did not actually describe the problematic situation, so add
back the extra reference and document it properly.

Fixes: fbd9a395 ("block: remove the extra kobject reference in bd_link_disk_holder")
Reported-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>