iptables: Example script for using socket iptables module

Wanted to add an example of DDoS protection using the socket
module, were we can aviod using conntrack.

It works, but this didn't turn out as well as I had hoped for.
Limitations lie both in the socket lookup __inet_lookup_listener()
and (mostly) in the hashlimit locking issue in dsthash_alloc_init().

Still the protection scales to approx 2Mpps, where I had hoped to see
between 4-5Mpps (on my test system Xeon X5550 10G-ixgbe).

Signed-off-by: Jesper Dangaard Brouer <netoptimizer@brouer.com>