xen-blkback: use bdev_discard_alignment

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2083917



commit 0000f2f7
Author: Christoph Hellwig <hch@lst.de>
Date:   Mon Apr 18 06:53:14 2022 +0200

    xen-blkback: use bdev_discard_alignment

    Use bdev_discard_alignment to calculate the correct discard alignment
    offset even for partitions instead of just looking at the queue limit.

    Also switch to use bdev_discard_granularity to get rid of the last direct
    queue reference in xen_blkbk_discard.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
    Link: https://lore.kernel.org/r/20220418045314.360785-12-hch@lst.de


    [axboe: fold in 'q' removal as it's now unused]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>