xfrm: interface: support IP6IP6 and IP6IP tunnels processing with .cb_handler

Similar to ip6_vti, IP6IP6 and IP6IP tunnels processing can easily
be done with .cb_handler for xfrm interface.

v1->v2:
  - no change.
v2-v3:
  - enable it only when CONFIG_INET6_XFRM_TUNNEL is defined, to fix
    the build error, reported by kbuild test robot.

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