selftests/bpf: Use start_server_addr in sockopt_inherit

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

Add a helper custom_cb() to set SOL_CUSTOM sockopt looply, set it to
post_socket_cb pointer of struct network_helper_opts, and pass it to
start_server_addr().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/687af66f743a0bf15cdba372c5f71fe64863219e.1714907662.git.tanggeliang@kylinos.cn


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