dm-bufio: fix wrong count calculation in dm_bufio_issue_discard

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: 6fbeb0048e6b ("dm bufio: implement discard")
Cc: stable@vger.kernel.org