block: Don't call ROUND_UP with negative values The behaviour of the ROUND_UP macro with negative numbers isn't obvious. It happens to do the right thing in this please, but better avoid it. Suggested-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com>