selftests/bpf: Use start_server_str in sk_lookup

selftests/bpf: Use start_server_str in sk_lookup

This patch uses public helper start_server_str() to simplify make_server()
in prog_tests/sk_lookup.c.

Add a callback setsockopts() to do all sockopts, set it to post_socket_cb
pointer of struct network_helper_opts. And add a new struct cb_opts to save
the data needed to pass to the callback. Then pass this network_helper_opts
to start_server_str().

Also use ASSERT_OK_FD() to check fd returned by start_server_str().

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/5981539f5591d2c4998c962ef2bf45f34c940548.1720515893.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>