selftests/bpf: Drop get_port in test_tcp_check_syncookie

The arguments "addr" and "len" of run_test() have dropped. This makes
function get_port() useless. Drop it from test_tcp_check_syncookie_user.c.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/a9b5c8064ab4cbf0f68886fe0e4706428b8d0d47.1714907662.git.tanggeliang@kylinos.cn


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