ip6_vti: support IP6IP6 tunnel processing with .cb_handler

Similar to IPIP tunnel's processing, this patch is to support
IP6IP6 tunnel processing with .cb_handler.

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>