netlink: Warn on unordered or illegal nla_nest_cancel() or nlmsg_cancel()

Calling nla_nest_cancel() in a different order as the nesting was
built up can lead to negative offsets being calculated which
results in skb_trim() being called with an underflowed unsigned
int. Warn if mark < skb->data as it's definitely a bug.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -520,8 +520,10 @@ static inline void *nlmsg_get_pos(struct sk_buff *skb)
  */
 static inline void nlmsg_trim(struct sk_buff *skb, const void *mark)
 {
-	if (mark)
+	if (mark) {
+		WARN_ON((unsigned char *) mark < skb->data);
 		skb_trim(skb, (unsigned char *) mark - skb->data);
+	}
 }
 
 /**