datapath: Add support for lwtunnel

datapath: Add support for lwtunnel

Following patch adds support for lwtunnel to OVS datapath.
With this change OVS datapath detect lwtunnel support and
make use of new APIs if available. On older kernel where the
support is not there the backported tunnel modules are used.
These backported tunnel devices acts as lwtunnel devices.
I tried to keep backported module same as upstream for easier
bug-fix backport. Since STT and LISP are not upstream OVS
always needs to use respective modules from tunnel compat layer.
To make it work on kernel 4.3 I have converted STT and LISP
modules to lwtunnel API model.

lwtunnel make use of skb-dst to pass tunnel information to the
tunnel module. On older kernel this is not possible. So the in
case of old kernel metadata ref is stored in OVS_CB and direct
call to tunnel transmit function is made by respective tunnel
vport modules. Similarly on receive side tunnel recv directly
call netdev-vport-receive to pass the skb to OVS.

Major backported components include:
Geneve, GRE, VXLAN, ip_tunnel, udp-tunnels GRO.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Joe Stringer <joe@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>