selftests/bpf: Selftest for sys_bind post-hooks.

Add selftest for attach types `BPF_CGROUP_INET4_POST_BIND` and
`BPF_CGROUP_INET6_POST_BIND`.

The main things tested are:
* prog load behaves as expected (valid/invalid accesses in prog);
* prog attach behaves as expected (load- vs attach-time attach types);
* `BPF_CGROUP_INET_SOCK_CREATE` can be attached in a backward compatible
  way;
* post-hooks return expected result and errno.

Example:
  # ./test_sock
  Test case: bind4 load with invalid access: src_ip6 .. [PASS]
  Test case: bind4 load with invalid access: mark .. [PASS]
  Test case: bind6 load with invalid access: src_ip4 .. [PASS]
  Test case: sock_create load with invalid access: src_port .. [PASS]
  Test case: sock_create load w/o expected_attach_type (compat mode) ..
  [PASS]
  Test case: sock_create load w/ expected_attach_type .. [PASS]
  Test case: attach type mismatch bind4 vs bind6 .. [PASS]
  Test case: attach type mismatch bind6 vs bind4 .. [PASS]
  Test case: attach type mismatch default vs bind4 .. [PASS]
  Test case: attach type mismatch bind6 vs sock_create .. [PASS]
  Test case: bind4 reject all .. [PASS]
  Test case: bind6 reject all .. [PASS]
  Test case: bind6 deny specific IP & port .. [PASS]
  Test case: bind4 allow specific IP & port .. [PASS]
  Test case: bind4 allow all .. [PASS]
  Test case: bind6 allow all .. [PASS]
  Summary: 16 PASSED, 0 FAILED

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>