For recent kernels, the peer-ifindex of veths is reported as
parent (IFA_LINK). Prefer that over the ethtool lookup.

For one, this avoids the extra ethtool call which has the
downside of sidestepping the platform cache. Also, looking
up the peer-ifindex in ethtool does not report whether the
peer lifes in another netns (NM_PLATFORM_LINK_OTHER_NETNS).

Only use ethtool as fallback for older kernels.