Make network_prefix return the number of bytes parsed.
Make network_prefix return the number of bytes parsed.

We used to recompute this in parse_packet, which is clearly the
wrong place.

wlanslovenija/babeld,tcatm/babeld,jech/babeld,sudomesh/babeld,jech/babeld,Drooids/babeld,dtaht/babeld-shortrtt-metrics,woniullb/babeld,wlanslovenija/babeld,Gwendocg/babeldToS,boutier/babeld,Gwendocg/babeldToS