selftests: net: add UDP GRO forwarding self-tests

Create a bunch of virtual topologies and verify that
NETIF_F_GRO_FRAGLIST or NETIF_F_GRO_UDP_FWD-enabled
devices aggregate the ingress packets as expected.
Additionally check that the aggregate packets are
segmented correctly when landing on a socket

Also test SKB_GSO_FRAGLIST and SKB_GSO_UDP_L4 aggregation
on top of UDP tunnel (vxlan)

v1 -> v2:
 - hopefully clarify the commit message
 - moved the overlay network ipv6 range into the 'documentation'
   reserved range (Willem)

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>