Filter netlink responses to only consider default routes by checking the
destination prefix length (rtm_dst_len == 0).
Previously, we selected the first route with an RTA_GATEWAY attribute,
which for IPv6 often resulted in choosing a non-default route instead of
the actual default.
This caused occasional PCP port mapping failures because a gateway for a
non-default route was selected.
In almost all cases (the only exception is `getifaddrs`), we know the
size of the data passed into SetSockAddr, so we can check this to be
what is expected.
