cls_u32: signedness bug

skb_headroom() is unsigned so "skb_headroom(skb) + toff" is also
unsigned and can't be less than zero.  This test was added in 66d50d25:
"u32: negative offset fix"  It was supposed to fix a regression.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_u32.c

@@ -137,7 +137,7 @@ next_knode:
 			int toff = off + key->off + (off2 & key->offmask);
 			__be32 *data, _data;
 
-			if (skb_headroom(skb) + toff < 0)
+			if (skb_headroom(skb) + toff > INT_MAX)
 				goto out;
 
 			data = skb_header_pointer(skb, toff, 4, &_data);