openvswitch: gre tunneling support.

Pravin B Shelar says:

====================
Following patch series adds support for gre tunneling.
First six patches extend kernel gre and ip_tunnel modules
api so that there is more code sharing between gre modules
and ovs. Rest of patches adds ovs tunneling infrastructre
and gre protocol vport.

V2 fixes two patches according to comments from Jesse.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>