net: BUG instead of corrupting memory in pskb_expand_head

If the caller of pskb_expand_head specifies a negative nhead
we'll silently overwrite other people's memory.  This patch
makes it BUG instead.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -738,6 +738,8 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 #endif
 	long off;
 
+	BUG_ON(nhead < 0);
+
 	if (skb_shared(skb))
 		BUG();