brd: remove the sector variable in brd_submit_bio

The bvec iter iterates over the sector already, no need to duplicate the
work.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20250428141014.2360063-3-hch@lst.de


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