vti6: Support inter address family tunneling.

With this patch we can tunnel ipv4 traffic via a vti6
interface. A vti6 interface can now have an ipv4 address
and ipv4 traffic can be routed via a vti6 interface.
The resulting traffic is xfrm transformed and tunneled
through ipv6 if matching IPsec policies and states are
present.

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