ip_vti: not register vti_ipip_handler twice

An xfrm_tunnel object is linked into the list when registering,
so vti_ipip_handler can not be registered twice, otherwise its
next pointer will be overwritten on the second time.

So this patch is to define a new xfrm_tunnel object to register
for AF_INET6.

Fixes: e6ce6457 ("ip_vti: support IPIP6 tunnel processing")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>