act_nat: fix wild pointer

pskb_may_pull() may change skb pointers, so adjust icmph after pskb_may_pull().
Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_nat.c

@@ -218,6 +218,7 @@ static int tcf_nat(struct sk_buff *skb, struct tc_action *a,
 		if (!pskb_may_pull(skb, ihl + sizeof(*icmph) + sizeof(*iph)))
 			goto drop;
 
+		icmph = (void *)(skb_network_header(skb) + ihl);
 		iph = (void *)(icmph + 1);
 		if (egress)
 			addr = iph->daddr;