block: use bdev_alignment_offset in part_alignment_offset_show

Replace the open coded offset calculation with the proper helper.
This is an ABI change in that the -1 for a misaligned partition is
properly propagated, which can be considered a bug fix and matches
what is done on the whole device.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220415045258.199825-17-hch@lst.de


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