selftests/bpf: Use post_socket_cb in connect_to_fd_opts

selftests/bpf: Use post_socket_cb in connect_to_fd_opts

Since the post_socket_cb() callback is added in struct network_helper_opts,
it's make sense to use it not only in __start_server(), but also in
connect_to_fd_opts(). Then it can be used to set TCP_CONGESTION sockopt.

Add a "void *" type member cb_opts into struct network_helper_opts, and add
a new struct named cb_opts in prog_tests/bpf_tcp_ca.c, then cc can be moved
into struct cb_opts from network_helper_opts. Define a new callback cc_cb()
to set TCP_CONGESTION sockopt, and set it to post_socket_cb pointer of opts.
Define a new cb_opts cubic, set it to cb_opts of opts. Pass this opts to
connect_to_fd_opts() in test_dctcp_fallback().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/b512bb8d8f6854c9ea5c409b69d1bf37c6f272c6.1716638248.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>