crazy udp output with reverse mode

* Version of iperf3: 3.9

* Hardware: intel x86_64

* Operating system (and distribution, if any): slackware 14.2

# Bug Report

I'm getting these crazy output when I run iperf3.9 udp traffic and reverse mode (-R)
10.24.209.252 host is running on a 10Gbit/s NIC, while the 10.68.64.37 is running on a 1Gbit/s NIC
```
iperf3 -V -c 10.68.64.37 -u -b 800M -t 10 -R
iperf 3.9
Linux FRPARDPADMSVI 2.6.32-754.6.3.el6.x86_64 #1 SMP Tue Sep 18 10:29:08 EDT 2018 x86_64
Control connection MSS 1448
Setting UDP block size to 1448
Time: Mon, 24 Aug 2020 09:54:43 GMT
Connecting to host 10.68.64.37, port 5201
Reverse mode, remote host 10.68.64.37 is sending
Cookie: sju3p5e5vt3csydm72y6whzxwjewnbnf4al7
Target Bitrate: 800000000
[ 5] local 10.24.209.252 port 36104 connected to 10.68.64.37 port 5201
Starting Test: protocol: UDP, 1 streams, 1448 byte blocks, omitting 0 seconds, 10 second test, tos 0
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-1.00 sec 95.4 MBytes 800 Mbits/sec 0.011 ms 2622/71690 (3.7%)
[ 5] 1.00-2.00 sec 94.6 MBytes 793 Mbits/sec 0.013 ms 573/69059 (0.83%)
[ 5] 2.00-3.00 sec 98.0 MBytes 822 Mbits/sec 0.014 ms -1919/69045 (-2.8%)
[ 5] 3.00-4.00 sec 147 MBytes 1.24 Gbits/sec 0.007 ms 698/69091 (1%)
[ 5] 4.00-5.00 sec 132 MBytes 1.11 Gbits/sec 0.008 ms -1465/69045 (-2.1%)
[ 5] 5.00-6.00 sec 126 MBytes 1.06 Gbits/sec 0.017 ms -165/68877 (-0.24%)
[ 5] 6.00-7.00 sec 122 MBytes 1.02 Gbits/sec 0.196 ms -344/69263 (-0.5%)
[ 5] 7.00-8.00 sec 139 MBytes 1.17 Gbits/sec 0.140 ms 0/68939 (0%)
[ 5] 8.00-9.00 sec 164 MBytes 1.37 Gbits/sec 0.007 ms 0/69173 (0%)
[ 5] 9.00-10.00 sec 167 MBytes 1.40 Gbits/sec 0.026 ms 0/69058 (0%)
- - - - - - - - - - - - - - - - - - - - - - - - -
Test Complete. Summary Results:
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-10.07 sec 960 MBytes 800 Mbits/sec 0.000 ms 0/695461 (0%) sender
[SUM] 0.0-10.1 sec 362892 datagrams received out-of-order
[ 5] 0.00-10.00 sec 1.25 GBytes 1.08 Gbits/sec 0.026 ms 0/693240 (0%) receiver

iperf Done.
```

```
iperf3 -V -c 10.68.64.37 -u -b 800M -t 10 -R
iperf 3.9
Linux FRPARDPADMSVI 2.6.32-754.6.3.el6.x86_64 #1 SMP Tue Sep 18 10:29:08 EDT 2018 x86_64
Control connection MSS 1448
Setting UDP block size to 1448
Time: Mon, 24 Aug 2020 09:55:34 GMT
Connecting to host 10.68.64.37, port 5201
Reverse mode, remote host 10.68.64.37 is sending
Cookie: cz3f4ihzh4zwyussouvlbtyex45nmog42xrv
Target Bitrate: 800000000
[ 5] local 10.24.209.252 port 36804 connected to 10.68.64.37 port 5201
Starting Test: protocol: UDP, 1 streams, 1448 byte blocks, omitting 0 seconds, 10 second test, tos 0
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-1.00 sec 178 MBytes 1.50 Gbits/sec 0.009 ms 0/112027 (0%)
[ 5] 1.00-2.00 sec 189 MBytes 1.59 Gbits/sec 0.018 ms 0/26066 (0%)
[ 5] 2.00-3.00 sec 189 MBytes 1.59 Gbits/sec 0.043 ms 0/69056 (0%)
[ 5] 3.00-4.00 sec 190 MBytes 1.60 Gbits/sec 0.014 ms 0/69054 (0%)
[ 5] 4.00-5.00 sec 189 MBytes 1.59 Gbits/sec 0.014 ms 0/69056 (0%)
[ 5] 5.00-6.00 sec 190 MBytes 1.59 Gbits/sec 0.032 ms 0/69050 (0%)
[ 5] 6.00-7.00 sec 188 MBytes 1.58 Gbits/sec 0.010 ms 0/69086 (0%)
[ 5] 7.00-8.00 sec 188 MBytes 1.58 Gbits/sec 0.012 ms 0/69041 (0%)
[ 5] 8.00-9.00 sec 188 MBytes 1.58 Gbits/sec 0.081 ms 0/68998 (0%)
[ 5] 9.00-10.00 sec 188 MBytes 1.58 Gbits/sec 0.042 ms 0/69117 (0%)
- - - - - - - - - - - - - - - - - - - - - - - - -
Test Complete. Summary Results:
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-10.03 sec 957 MBytes 800 Mbits/sec 0.000 ms 0/692797 (0%) sender
[SUM] 0.0-10.0 sec 787254 datagrams received out-of-order
[ 5] 0.00-10.00 sec 1.84 GBytes 1.58 Gbits/sec 0.042 ms 0/690551 (0%) receiver

iperf Done.
```

Edit: Reformatted program output for legibility. --bmah.