bpf: selftest: Use bpf_skc_to_tcp_sock() in the sock_fields test

bpf: selftest: Use bpf_skc_to_tcp_sock() in the sock_fields test

This test uses bpf_skc_to_tcp_sock() to get a kernel tcp_sock ptr "ktp".
Access the ktp->lsndtime and also pass ktp to bpf_sk_storage_get().

It also exercises the bpf_sk_cgroup_id() and bpf_sk_ancestor_cgroup_id()
with the "ktp".  To do that, a parent cgroup and a child cgroup are
created.  The bpf prog is attached to the child cgroup.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200925000446.3858975-1-kafai@fb.com