selftests/bpf: Add post_socket_cb for network_helper_opts

__start_server() sets SO_REUSPORT through setsockopt() when the parameter
'reuseport' is set. This patch makes it more flexible by adding a function
pointer post_socket_cb into struct network_helper_opts. The
'const struct post_socket_opts *cb_opts' args in the post_socket_cb is
for the future extension.

The 'reuseport' parameter can be dropped.
Now the original start_reuseport_server() can be implemented by setting a
newly defined reuseport_cb() function pointer to post_socket_cb filed of
struct network_helper_opts.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/470cb82f209f055fc7fb39c66c6b090b5b7ed2b2.1714907662.git.tanggeliang@kylinos.cn


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