ipv6: fix a potential use after free in ip6_offload.c

pskb_may_pull() maybe change skb->data and make opth pointer oboslete,
so set the opth again

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_offload.c

@@ -46,6 +46,7 @@ static int ipv6_gso_pull_exthdrs(struct sk_buff *skb, int proto)
 		if (unlikely(!pskb_may_pull(skb, len)))
 			break;
 
+		opth = (void *)skb->data;
 		proto = opth->nexthdr;
 		__skb_pull(skb, len);
 	}