selftests/bpf: Fix const'ness in cgroup_helpers

Lack of const in cgroup helpers signatures forces to write ugly client
code. Fix it.

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