selftests/bpf: bpf tunnel test.

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

In summary, 'test_tunnel_kern.o' contains the following bpf program:
  GRE: gre_set_tunnel, gre_get_tunnel
  IP6GRE: ip6gretap_set_tunnel, ip6gretap_get_tunnel
  ERSPAN: erspan_set_tunnel, erspan_get_tunnel
  IP6ERSPAN: ip4ip6erspan_set_tunnel, ip4ip6erspan_get_tunnel
  VXLAN: vxlan_set_tunnel, vxlan_get_tunnel
  IP6VXLAN: ip6vxlan_set_tunnel, ip6vxlan_get_tunnel
  GENEVE: geneve_set_tunnel, geneve_get_tunnel
  IP6GENEVE: ip6geneve_set_tunnel, ip6geneve_get_tunnel
  IPIP: ipip_set_tunnel, ipip_get_tunnel
  IP6IP: ipip6_set_tunnel, ipip6_get_tunnel,
         ip6ip6_set_tunnel, ip6ip6_get_tunnel
  XFRM: xfrm_get_state

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/Makefile

@@ -32,7 +32,7 @@ TEST_GEN_FILES = test_pkt_access.o test_xdp.o test_l4lb.o test_tcp_estats.o test
 	test_l4lb_noinline.o test_xdp_noinline.o test_stacktrace_map.o \
 	sample_map_ret0.o test_tcpbpf_kern.o test_stacktrace_build_id.o \
 	sockmap_tcp_msg_prog.o connect4_prog.o connect6_prog.o test_adjust_tail.o \
-	test_btf_haskv.o test_btf_nokv.o test_sockmap_kern.o
+	test_btf_haskv.o test_btf_nokv.o test_sockmap_kern.o test_tunnel_kern.o
 
 # Order correspond to 'make run_tests' order
 TEST_PROGS := test_kmod.sh \
@@ -40,7 +40,8 @@ TEST_PROGS := test_kmod.sh \
 	test_xdp_redirect.sh \
 	test_xdp_meta.sh \
 	test_offload.py \
-	test_sock_addr.sh
+	test_sock_addr.sh \
+	test_tunnel.sh
 
 # Compile but not part of 'make run_tests'
 TEST_GEN_PROGS_EXTENDED = test_libbpf_open test_sock_addr