u32: negative offset fix

It was possible to use a negative offset in a u32 match to reference
the ethernet header or other parts of the link layer header.
This fixes the regression caused by:

commit fbc2e7d9
Author: Changli Gao <xiaosuo@gmail.com>
Date:   Wed Jun 2 07:32:42 2010 -0700

    cls_u32: use skb_header_pointer() to dereference data safely
Signed-off-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/cls_u32.c

@@ -134,10 +134,12 @@ static int u32_classify(struct sk_buff *skb, struct tcf_proto *tp, struct tcf_re
 #endif
 
 		for (i = n->sel.nkeys; i>0; i--, key++) {
-			unsigned int toff;
+			int toff = off + key->off + (off2 & key->offmask);
 			__be32 *data, _data;
 
-			toff = off + key->off + (off2 & key->offmask);
+			if (skb_headroom(skb) + toff < 0)
+				goto out;
+
 			data = skb_header_pointer(skb, toff, 4, &_data);
 			if (!data)
 				goto out;