selftests/bpf: Use connect_to_fd in sockopt_inherit

This patch uses public helper connect_to_fd() exported in network_helpers.h
instead of the local defined function connect_to_server() in
prog_tests/sockopt_inherit.c. This can avoid duplicate code.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/71db79127cc160b0643fd9a12c70ae019ae076a1.1714907662.git.tanggeliang@kylinos.cn


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