selftests: add a selftest for big tcp

This test runs on the client-router-server topo, and monitors the traffic
on the RX devices of router and server while sending BIG TCP packets with
netperf from client to server. Meanwhile, it changes 'tso' on the TX devs
and 'gro' on the RX devs. Then it checks if any BIG TCP packets appears
on the RX devs with 'ip/ip6tables -m length ! --length 0:65535' for each
case.

Note that we also add tc action ct in link1 ingress to cover the ipv6
jumbo packets process in nf_ct_skb_network_trim() of nf_conntrack_ovs.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Florian Westphal <fw@strlen.de>