vti4: Support inter address family tunneling.

With this patch we can tunnel ipv6 traffic via a vti4
interface. A vti4 interface can now have an ipv6 address
and ipv6 traffic can be routed via a vti4 interface.
The resulting traffic is xfrm transformed and tunneled
throuhg ipv4 if matching IPsec policies and states are
present.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>