iperf_sctp: apply default socket buffer to fix slow single-CPU throughput Linux SCTP

[nshopik]

Unlike TCP, it does not auto-tune SO_RCVBUF/SO_SNDBUF (there is no equivalent of tcp_moderate_rcvbuf). Without an explicit setting the per-association receive window collapses to ~40 KB, which on single-CPU systems gates single-stream loopback throughput to roughly 1 Gbps even after the delayed-SACK fix from #1815: sender and softirq compete for the same core, so the small rwnd is never reopened in time.

Apply DEFAULT_SCTP_SOCKBUF (512 KB) to SO_RCVBUF and SO_SNDBUF on the client connect socket, the server listen socket, and the per- association accept socket whenever --window is not given. The kernel silently caps at net.core.{r,w}mem_max where smaller, and the explicit --window path is unchanged. Failures on the default path are non-fatal (best-effort, not user-requested).

Measured on Debian 13 (kernel 6.12.48), loopback, single stream, 30 s runs, median of 3:

1-vCPU box
  TCP default          : 25.5 Gbps (unchanged)
  SCTP default before  :  0.94 Gbps
  SCTP default after   : 25.5 Gbps  (~27x)

2-vCPU box
  SCTP default before/after: ~22 Gbps (within run-to-run variance)

The gap only manifests when sender and softirq share a CPU; on multi- core systems the kernel already works around the small rwnd and the patch is a no-op there.

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: master

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[bmah888]

Thanks for the PR! Will test this.