Add BPF kernel module to 20.04 and 22.04 images

[Dagmawi Biru]

LXD image provided in the ubuntu: remotes do not contain BPF kernel module.

BPF module is present in the stock linux-generic versions of the kernel package
but *-kvm kernel does not appear to be built with it -- this is a request for its inclusion.

Repro:
---
1. Deploy a 20.04 or 22.04 release image (ubuntu:focal or ubuntu:jammy should suffice) VM on LXD (not container)
2. Observe uname -r to see that the -kvm variant of the ubuntu kernel is running
3. Check to see if BPFilter module is available:

# find /lib/modules/$(uname -r)/ -type f -name '*bpfilter.ko'

There are use cases where the BPF module is still used and it may be a while before users
are able to transition to 24.04+ which I understand transitions to using the generic kernel.