Fix GSO / GRO auto detection when IPv6 is disabled.

[stormshield-damiend]

One of my Linux test machine has IPv6 disabled. While doing some bench and test on it, i noticed that GSO / GRO were not enabled. Here is a small fix to fallback to an IPv4 loopback in case it failed on IPv6 loopback.

[Ralith]

Thanks!