datapath: Add Stateless TCP Tunneling protocol.

datapath: Add Stateless TCP Tunneling protocol.

The Stateless TCP Tunnel (STT) protocol encapsulates traffic in
IPv4/TCP packets.
STT uses TCP segmentation offload available in most of NIC. On
packet xmit STT driver appends STT header along with TCP header
to the packet. For GSO packet GSO parameters are set according
to tunnel configuration and packet is handed over to networking
stack. This allows use of segmentation offload available in NICs

The protocol is documented at
http://www.ietf.org/archive/id/draft-davie-stt-06.txt

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>