tunneling: Add userspace tunnel support for Geneve.

tunneling: Add userspace tunnel support for Geneve.

This adds basic userspace dataplane support for the Geneve
tunneling protocol. The rest of userspace only has the ability
to handle Geneve without options and this follows that pattern
for the time being. However, when the rest of userspace is updated
it should be easy to extend the dataplane as well.

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