sit: reload iphdr in ipip6_rcv

Since iptunnel_pull_header() can call pskb_may_pull(),
we must reload any pointer that was related to skb->head.

Fixes: a09a4c8dd1 ("tunnels: Remove encapsulation offloads on decap")
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -657,6 +657,7 @@ static int ipip6_rcv(struct sk_buff *skb)
 		if (iptunnel_pull_header(skb, 0, htons(ETH_P_IPV6),
 		    !net_eq(tunnel->net, dev_net(tunnel->dev))))
 			goto out;
+		iph = ip_hdr(skb);
 
 		err = IP_ECN_decapsulate(iph, skb);
 		if (unlikely(err)) {