Limit UDP GSO number of segments in a message to the allowed value

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master 3.21+

• Issues fixed (if any): GSO broken for a length value lower than 508 #2032

• Brief description of code changes (suitable for use as a commit message):

Suggested fix to PR #1925 to limit the number of segments in UDP GSO message to 128, which is the maximum allowed. The problem was with messages length 507 or less, as 507 is the maximum length where MAX_UDP_BLOCKSIZE/length >= 129.

The limit GSO_MAX_DG_IN_BF is defined as a constant as I didn't find a reliable way to determine it dynamically. From what I found, the value is defined in Linux as UDP_MAX_SEGMENTS. Although it seems that in some (newer?) Linux distributions it is defined in udp.h, as in here, at least in WSL Linux that I use it is not defined in a header file, and probably it is defined in udpgso.c like in here.

[abajk]

Now it works fine even with the minimum UDP packet size :D

[bmah888]

Thanks for the PR! This looks pretty reasonable (I haven't done any testing).

[bmah888]

I was able to successfully reproduce the original (broken) behavior of #2032 without the patch, and verified that data actually gets sent and recorded with the patch. Looks good to me!