selftests/bpf: Add backlog for network_helper_opts

Some callers expect __start_server() helper to pass their own "backlog"
value to listen() instead of the default of 1. So this patch adds struct
member "backlog" for network_helper_opts to allow callers to set "backlog"
value via start_server_str() helper.

listen(fd, 0 /* backlog */) can be used to enforce syncookie. Meaning
backlog 0 is a legit value.

Using 0 as a default and changing it to 1 here is fine. It makes the test
program easier to write for the common case. Enforcing syncookie mode by
using backlog 0 is a niche use case but it should at least have a way for
the caller to do that. Thus, -ve backlog value is used here for the
syncookie use case. Please see the comment in network_helpers.h for
the details.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/1660229659b66eaad07aa2126e9c9fe217eba0dd.1720515893.git.tanggeliang@kylinos.cn


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