selftests/bpf: Use start_server_addr in test_sock_addr

Include network_helpers.h in test_sock_addr.c, use the newly added public
helper start_server_addr() instead of the local defined function
start_server(). This can avoid duplicate code.

In order to use functions defined in network_helpers.c in test_sock_addr.c,
Makefile needs to be updated and <Linux/err.h> needs to be included in
network_helpers.h to avoid compilation errors.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/3101f57bde5502383eb41723c8956cc26be06893.1713868264.git.tanggeliang@kylinos.cn


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