xfrm: add is_ipip to struct xfrm_input_afinfo

This patch is to add a new member is_ipip to struct xfrm_input_afinfo,
to allow another group family of callback functions to be registered
with is_ipip set.

This will be used for doing a callback for struct xfrm(6)_tunnel of
ipip/ipv6 tunnels in xfrm_input() by calling xfrm_rcv_cb(), which is
needed by ipip/ipv6 tunnels' support in ip(6)_vti and xfrm interface
in the next patches.

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