-M has no effect on packet size during data packets initiated from iperf3 client

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# Context
In iperf3 application though the MSS is negotiated based on -M option during TCP SYN, but the data packets generated by iperf3 client are much bigger in size. Looks like it is not honoring the -M option to set MSS

* Version of iperf3:
iperf3 version 3.5

* Hardware:
General purpose server
* Operating system (and distribution, if any):
Linux
_Please note: iperf3 is supported on Linux, FreeBSD, and macOS.
Support may be provided on a best-effort basis to other UNIX-like
platforms. We cannot provide support for building and/or running
iperf3 on Windows, iOS, or Android._

* Other relevant information (for example, non-default compilers,
libraries, cross-compiling, etc.):

_Please fill out one of the "Bug Report" or "Enhancement Request"
sections, as appropriate. Note that submissions of bug fixes, new
features, etc. should be done as a pull request at
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# Bug Report

* Expected Behavior

* Actual Behavior

* Steps to Reproduce

* Possible Solution

# Enhancement Request

* Current behavior

* Desired behavior

* Implementation notes