benchmarks/iperf: Add patch to fix large values of -P by setting a large listen(2) backlog.

[skozlov]

Summary

Because, by default, the listening socket backlog is set to a very low value (5) - if one uses big number of parallel connections - many connections can fail. This patch increases the backlog to a biggest possible value.

This is the same fix as made for iperf3 here:
https://github.com/esnet/iperf/pull/693/commits/20fdcbe2fd1440741d2fd641287a4784f26a1c9d

This is a temporary fix, while the upstream patch is being reviewed:
https://sourceforge.net/p/iperf2/tickets/43/

Sponsored by: Intel Corporation

Test Plan

Poudriere log:

iperf-2.0.12_1_poudriere.log130 KBDownload

Compile tested on:
10.4-RELEASE-p9
11.1-RELEASE-p11
11.2-RELEASE
12.0-CURRENT (1200069)

[skozlov]

Adding port maintainer as a blocking reviewer.

[skozlov]

Old and no longer relevant