ip_vti: support IPIP6 tunnel processing

For IPIP6 tunnel processing, the functions called will be the
same as that for IPIP tunnel's. So reuse it and register it
with family == AF_INET6.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>