brd: fix aligned_sector from brd_do_discard()

The calculation is just wrong, fix it by round_up().

Fixes: 9ead7efc ("brd: implement discard support")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250506061756.2970934-3-yukuai1@huaweicloud.com


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