selftests/bpf: Add a new cgroup helper get_classid_cgroup_id()

Introduce a new helper function to retrieve the cgroup ID from a net_cls
cgroup directory.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20231111090034.4248-5-laoar.shao@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>