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>
