bpf: Add test_sock_fields for skb->sk and bpf_tcp_sock

This patch adds a C program to show the usage on
skb->sk and bpf_tcp_sock.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>