virtio_blk: fix the discard_granularity and discard_alignment queue limits

The discard_alignment queue limit is named a bit misleading means the
offset into the block device at which the discard granularity starts.

On the other hand the discard_sector_alignment from the virtio 1.1 looks
similar to what Linux uses as discard granularity (even if not very well
described):

  "discard_sector_alignment can be used by OS when splitting a request
   based on alignment. "

And at least qemu does set it to the discard granularity.

So stop setting the discard_alignment and use the virtio
discard_sector_alignment to set the discard granularity.

Fixes: 1f23816b8e ("virtio_blk: add discard and write zeroes support")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20220418045314.360785-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/virtio_blk.c

@@ -867,11 +867,12 @@ static int virtblk_probe(struct virtio_device *vdev)
 		blk_queue_io_opt(q, blk_size * opt_io_size);
 
 	if (virtio_has_feature(vdev, VIRTIO_BLK_F_DISCARD)) {
-		q->limits.discard_granularity = blk_size;
-
 		virtio_cread(vdev, struct virtio_blk_config,
 			     discard_sector_alignment, &v);
-		q->limits.discard_alignment = v ? v << SECTOR_SHIFT : 0;
+		if (v)
+			q->limits.discard_granularity = v << SECTOR_SHIFT;
+		else
+			q->limits.discard_granularity = blk_size;
 
 		virtio_cread(vdev, struct virtio_blk_config,
 			     max_discard_sectors, &v);