block: use bdev_discard_alignment in part_discard_alignment_show

Use the bdev based alignment helper instead of open coding it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20220415045258.199825-21-hch@lst.de


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