econet: Fix crash in aun_incoming().

Unconditional use of skb->dev won't work here,
try to fetch the econet device via skb_dst()->dev
instead.

Suggested by Eric Dumazet.
Reported-by: NNelson Elhage <nelhage@ksplice.com>
Tested-by: NNelson Elhage <nelhage@ksplice.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/econet/af_econet.c

@@ -851,9 +851,13 @@ static void aun_incoming(struct sk_buff *skb, struct aunhdr *ah, size_t len)
 {
 	struct iphdr *ip = ip_hdr(skb);
 	unsigned char stn = ntohl(ip->saddr) & 0xff;
+	struct dst_entry *dst = skb_dst(skb);
+	struct ec_device *edev = NULL;
 	struct sock *sk = NULL;
 	struct sk_buff *newskb;
-	struct ec_device *edev = skb->dev->ec_ptr;
+
+	if (dst)
+		edev = dst->dev->ec_ptr;
 
 	if (! edev)
 		goto bad;