High rate of packet loss upgrading to kernel 5.3 and later.

[Alan Jowett]

Performing testing on Ubuntu 18.04 LTS and Ubuntu 20.04 LTS.

Setup:
ocserv built from https://gitlab.com/openconnect/ocserv.git @ 59bdd070b6b9f893d3f552b6a8895a5f4eb014da, running in Docker container built from Ubuntu 18.04 Docker image.

Test:
1000 clients connected using openconnect (https://gitlab.com/openconnect/openconnect) (clients spread across 54 VMs, each client running in a Docker container for network isolation)

Simulating 3 workloads:
1) IM traffic - (TCP, with server echoing the stream and client sending [1,1024] bytes every [2500,7500]ms)
2) Audio streaming - (UDP, with server sending 128kbps, sending packets 60 times per second)
3) web-browser - (Loading of 3mb web-page, with 11 elements requiring separate HTTP gets, reloading every 60 seconds.)

Results:
Ubuntu 18.04 / Linux kernel version 4.15.0-101-generic (buildd@lgw01-amd64-003)) - less than 1% packet loss.
Ubuntu 18.04 / Linux kernel version 5.3.0-1022-azure (buildd@lcy01-amd64-001)) - 9% packet loss.
Ubuntu 20.04 / Linux kernel version 5.4.0-33 - 9% packet loss.

Tested on both 18.04 and 20.04 VM running on Hyper-V and on a bare-metal install of Ubuntu 20.04 LTS.