selftests/bpf: Use connect_fd_to_fd in sk_lookup

This patch uses public helper connect_fd_to_fd() exported in
network_helpers.h instead of using getsockname() + connect() in
run_lookup_prog() in prog_tests/sk_lookup.c. This can simplify
the code.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/7077c277cde5a1864cdc244727162fb75c8bb9c5.1720515893.git.tanggeliang@kylinos.cn


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