xfrm: interface: not xfrmi_ipv6/ipip_handler twice

As we did in the last 2 patches for vti(6), this patch is to define a
new xfrm_tunnel object 'xfrmi_ipip6_handler' to register for AF_INET6,
and a new xfrm6_tunnel object 'xfrmi_ip6ip_handler' to register for
AF_INET.

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