bpf, selftests: Add test case for mixed cgroup v1/v2

Minimal selftest which implements a small BPF policy program to the
connect(2) hook which rejects TCP connection requests to port 60123
with EPERM. This is being attached to a non-root cgroup v2 path. The
test asserts that this works under cgroup v2-only and under a mixed
cgroup v1/v2 environment where net_classid is set in the former case.

Before fix:

  # ./test_progs -t cgroup_v1v2
  test_cgroup_v1v2:PASS:server_fd 0 nsec
  test_cgroup_v1v2:PASS:client_fd 0 nsec
  test_cgroup_v1v2:PASS:cgroup_fd 0 nsec
  test_cgroup_v1v2:PASS:server_fd 0 nsec
  run_test:PASS:skel_open 0 nsec
  run_test:PASS:prog_attach 0 nsec
  test_cgroup_v1v2:PASS:cgroup-v2-only 0 nsec
  run_test:PASS:skel_open 0 nsec
  run_test:PASS:prog_attach 0 nsec
  run_test:PASS:join_classid 0 nsec
  (network_helpers.c:219: errno: None) Unexpected success to connect to server
  test_cgroup_v1v2:FAIL:cgroup-v1v2 unexpected error: -1 (errno 0)
  #27 cgroup_v1v2:FAIL
  Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED

After fix:

  # ./test_progs -t cgroup_v1v2
  #27 cgroup_v1v2:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210913230759.2313-3-daniel@iogearbox.net