target: fix discard alignment on partitions

Use the proper bdev_discard_alignment helper that accounts for partition
offsets.

Fixes: c66ac9db ("[SCSI] target: Add LIO target core v4.0.0-rc6")
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/20220415045258.199825-4-hch@lst.de


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