selftests/bpf: Use start_server_addr in test_tcp_check_syncookie

Include network_helpers.h in test_tcp_check_syncookie_user.c, use
public helper start_server_addr() in it instead of the local defined
function start_server(). This can avoid duplicate code.

Add two helpers v6only_true() and v6only_false() to set IPV6_V6ONLY
sockopt to true or false, set them to post_socket_cb pointer of struct
network_helper_opts, and pass it to start_server_setsockopt().

In order to use functions defined in network_helpers.c, Makefile needs
to be updated too.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/e0c5324f5da84f453f47543536e70f126eaa8678.1714907662.git.tanggeliang@kylinos.cn


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>