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>

drivers/block/xen-blkback/xenbus.c

@@ -575,7 +575,6 @@ static void xen_blkbk_discard(struct xenbus_transaction xbt, struct backend_info
 	int err;
 	int state = 0;
 	struct block_device *bdev = be->blkif->vbd.bdev;
-	struct request_queue *q = bdev_get_queue(bdev);
 
 	if (!xenbus_read_unsigned(dev->nodename, "discard-enable", 1))
 		return;
@@ -583,14 +582,14 @@ static void xen_blkbk_discard(struct xenbus_transaction xbt, struct backend_info
 	if (bdev_max_discard_sectors(bdev)) {
 		err = xenbus_printf(xbt, dev->nodename,
 			"discard-granularity", "%u",
-			q->limits.discard_granularity);
+			bdev_discard_granularity(bdev));
 		if (err) {
 			dev_warn(&dev->dev, "writing discard-granularity (%d)", err);
 			return;
 		}
 		err = xenbus_printf(xbt, dev->nodename,
 			"discard-alignment", "%u",
-			q->limits.discard_alignment);
+			bdev_discard_alignment(bdev));
 		if (err) {
 			dev_warn(&dev->dev, "writing discard-alignment (%d)", err);
 			return;