selftests/bpf: Use start_server_addr in sk_lookup

selftests/bpf: Use start_server_addr in sk_lookup

This patch uses public helper start_server_addr() in udp_recv_send()
in prog_tests/sk_lookup.c to simplify the code.

And use ASSERT_OK_FD() to check fd returned by start_server_addr().

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/f11cabfef4a2170ecb66a1e8e2e72116d8f621b3.1720515893.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>