block: remove the extra gendisk reference in __blk_mq_register_dev

kobject_add already grabs a reference to the parent, no need to have
another one.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220628171850.1313069-5-hch@lst.de


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