af-packet: add support for eBPF cluster and filter

af-packet: add support for eBPF cluster and filter

This patch introduces the ebpf cluster mode. This mode is using
an extended BPF function that is loaded into the kernel and
provide the load balancing.

An example of cluster function is provided in the ebpf
subdirectory and provide ippair load balancing function.
This is a function which uses the same method as
the one used in autofp ippair to provide a symetrical
load balancing based on IP addresses.

A simple filter example allowing to drop IPv6 is added to the
source.

This patch also prepares the infrastructure to be able to load
and use map inside eBPF files. This will be used later for flow
bypass.