ip6_vti: not register vti_ipv6_handler twice

An xfrm6_tunnel object is linked into the list when registering,
so vti_ipv6_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 xfrm6_tunnel object to register
for AF_INET.

Fixes: 2ab110cb ("ip6_vti: support IP6IP tunnel processing")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>