Fix: dss-csum if a 64-bit dss option is present

dss_off points to the beginning of the dss-option. If a 64-bit dss is
present, we should skip the first 4 bytes, as they are added later with
the final call to csum_partial.

Reported-by: Erkrishna Khanal <erkrishna.khanal@gmail.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>