The patch migrates the original tests at samples/bpf/tcbpf2_kern.c
and samples/bpf/test_tunnel_bpf.sh to selftests. There are a couple
changes from the original:
1) add ipv6 vxlan, ipv6 geneve, ipv6 ipip tests
2) simplify the original ipip tests (remove iperf tests)
3) improve documentation
4) use bpf_ntoh* and bpf_hton* api