selftests/bpf: Fix a fd leak in an error path in network_helpers.c

In __start_server, it leaks a fd when setsockopt(SO_REUSEPORT) fails.
This patch fixes it.

Fixes: eed92afd ("bpf: selftest: Test batching and bpf_(get|set)sockopt in bpf tcp iter")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20230316000726.1016773-2-martin.lau@linux.dev