selftests/bpf: Drop struct post_socket_opts

selftests/bpf: Drop struct post_socket_opts

It's not possible to have one generic/common "struct post_socket_opts"
for all tests. It's better to have the individual test define its own
callback opts struct.

So this patch drops struct post_socket_opts, and changes the second
parameter of post_socket_cb as "void *" type.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/f8bda41c7cb9cb6979b2779f89fb3a684234304f.1716638248.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>