netpoll: fix incorrect access to skb data in __netpoll_rx

__netpoll_rx() doesnt properly handle skbs with small header

pskb_may_pull() or pskb_trim_rcsum() can change skb->data, we must
reload it.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -558,13 +558,14 @@ int __netpoll_rx(struct sk_buff *skb)
 	if (skb_shared(skb))
 		goto out;
 
-	iph = (struct iphdr *)skb->data;
 	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
 		goto out;
+	iph = (struct iphdr *)skb->data;
 	if (iph->ihl < 5 || iph->version != 4)
 		goto out;
 	if (!pskb_may_pull(skb, iph->ihl*4))
 		goto out;
+	iph = (struct iphdr *)skb->data;
 	if (ip_fast_csum((u8 *)iph, iph->ihl) != 0)
 		goto out;
 
@@ -579,6 +580,7 @@ int __netpoll_rx(struct sk_buff *skb)
 	if (pskb_trim_rcsum(skb, len))
 		goto out;
 
+	iph = (struct iphdr *)skb->data;
 	if (iph->protocol != IPPROTO_UDP)
 		goto out;