Hopefully fix IPv6 fwding perf by disabling GSO

Apparently GSO (Generic Segmentation Offload) doesn't
work quite right when forwarding IPv6 in a virtual
environment.  It provides negligible improvements
to performance anyway, so let's just disable it
outright.
Merge pull request #95 from fzylogic/master

Hopefully fix IPv6 fwding perf by disabling GSO