ipip: validate header length in ipip_tunnel_xmit

We need the same checks introduced by commit cb9f1b78
("ip: validate header length on virtual device xmit") for
ipip tunnel.

Fixes: cb9f1b78 ("ip: validate header length on virtual device xmit")
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>