selftests/bpf: Use post_socket_cb in start_server_str

This patch uses start_server_str() helper in test_dctcp_fallback() in
bpf_tcp_ca.c, instead of using start_server() and settcpca(). For
support opts in start_server_str() helper, opts->cb_opts needs to be
passed to post_socket_cb() in __start_server().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/414c749321fa150435f7fe8e12c80fec8b447c78.1716638248.git.tanggeliang@kylinos.cn


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