selftests/bpf: Fix tx_prog_fd values in test_sockmap

selftests/bpf: Fix tx_prog_fd values in test_sockmap

The values of tx_prog_fd in run_options() should not be 0, so set it as -1
in else branch, and test it using "if (tx_prog_fd > 0)" condition, not
"if (tx_prog_fd)" or "if (tx_prog_fd >= 0)".

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/08b20ffc544324d40939efeae93800772a91a58e.1716446893.git.tanggeliang@kylinos.cn