net: add debug info to __skb_pull()

While analyzing yet another syzbot report, I found the following
patch very useful. It allows to better understand what went wrong.

This debug info is only enabled if CONFIG_DEBUG_NET=y,
which is the case for syzbot builds.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -2696,7 +2696,14 @@ void *skb_pull(struct sk_buff *skb, unsigned int len);
 static inline void *__skb_pull(struct sk_buff *skb, unsigned int len)
 {
 	skb->len -= len;
-	BUG_ON(skb->len < skb->data_len);
+	if (unlikely(skb->len < skb->data_len)) {
+#if defined(CONFIG_DEBUG_NET)
+		skb->len += len;
+		pr_err("__skb_pull(len=%u)\n", len);
+		skb_dump(KERN_ERR, skb, false);
+#endif
+		BUG();
+	}
 	return skb->data += len;
 }