net: Add debug info to track down GSO checksum bug

I'm trying to track down why people're hitting the checksum warning
in skb_gso_segment.  As the problem seems to be hitting lots of
people and I can't reproduce it or locate the bug, here is a patch
to print out more details which hopefully should help us to track
this down.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1534,7 +1534,19 @@ struct sk_buff *skb_gso_segment(struct sk_buff *skb, int features)
 	skb->mac_len = skb->network_header - skb->mac_header;
 	__skb_pull(skb, skb->mac_len);
 
-	if (WARN_ON(skb->ip_summed != CHECKSUM_PARTIAL)) {
+	if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
+		struct net_device *dev = skb->dev;
+		struct ethtool_drvinfo info = {};
+
+		if (dev && dev->ethtool_ops && dev->ethtool_ops->get_drvinfo)
+			dev->ethtool_ops->get_drvinfo(dev, &info);
+
+		WARN(1, "%s: caps=(0x%lx, 0x%lx) len=%d data_len=%d "
+			"ip_summed=%d",
+		     info.driver, dev ? dev->features : 0L,
+		     skb->sk ? skb->sk->sk_route_caps : 0L,
+		     skb->len, skb->data_len, skb->ip_summed);
+
 		if (skb_header_cloned(skb) &&
 		    (err = pskb_expand_head(skb, 0, 0, GFP_ATOMIC)))
 			return ERR_PTR(err);