iperf3 3.5 TCP option -n not working

# Context

* Version of iperf3:
iperf3.5

* Hardware:
amd64 laptop with Intel i5 CPU,

* Operating system (and distribution, if any):
Running on QEMU based VMs
Host OS: Ubuntu 16.04.4LTS
Guest OS: Alpine Linux 3.7+testing

* 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._

# Bug Report

* Expected Behavior
iperf3 -c -n 1.8M should produce a data transaction with 1.8M bytes

* Actual Behavior
Dumping from the PCAP file and filtering the FIN/ACK packet:
```
time sport dport seq ack flags
0.19117 56562 5201 38 2904690 FA
```
Expecting something around 1887436 and not 2904690 in the ack number

* Steps to Reproduce
VM1 as server, `iperf3 -s` ,capture packets with tshark
VM2 as client, `iperf3 -c -n 1.8M`
Analyse TCP SEQ and ACK numbers

* Possible Solution
Yet N/A