selftests/bpf: Use start_server_addr in cls_redirect

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

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/13f336cb4c6680175d50bb963d9532e11528c758.1713427236.git.tanggeliang@kylinos.cn


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