dm-bufio: fix wrong count calculation in dm_bufio_issue_discard

block_to_sector converts a block number to a sector number and adds
c->start to the result. It is inappropriate to use this function for
converting the number of blocks to a number to sectors because c->start
would be incorrectly added to the result.

Luckily, the only target that uses dm_bufio_issue_discard is dm-ebs,
which sets c->start to 0, so this bug is latent.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Fixes: 6fbeb0048e ("dm bufio: implement discard")
Cc: stable@vger.kernel.org

drivers/md/dm-bufio.c

@@ -2238,7 +2238,9 @@ int dm_bufio_issue_discard(struct dm_bufio_client *c, sector_t block, sector_t c
 	struct dm_io_region io_reg = {
 		.bdev = c->bdev,
 		.sector = block_to_sector(c, block),
-		.count = block_to_sector(c, count),
+		.count = likely(c->sectors_per_block_bits >= 0) ?
+			count << c->sectors_per_block_bits :
+			count * (c->block_size >> SECTOR_SHIFT),
 	};
 
 	if (WARN_ON_ONCE(dm_bufio_in_request()))