ip_vti: support IPIP tunnel processing with .cb_handler

With tunnel4_input_afinfo added, IPIP tunnel processing in
ip_vti can be easily done with .cb_handler. So replace the
processing by calling ip_tunnel_rcv() with it.

v1->v2:
  - no change.
v2-v3:
  - enable it only when CONFIG_INET_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>