Automatic peer MAC detection for xdp-bench redirect to veth

[tohojo]

When running xdp-bench redirect redirecting into a veth device, it would be
useful to automatically detect the peer MAC address and set that as the
destination MAC. This would require some way to find the veth peer and query its
MAC, which is complicated by the fact that the peer can be in a different
namespace.

[netoptimizer]

As per discussion with @donaldh and @maryamtahhan