net: add a temporary sanity check in skb_orphan()

David suggested to add a BUG_ON() to catch if some layer
sets skb->sk pointer without a corresponding destructor.

As skb can sit in a queue, it's mandatory to make sure the
socket cannot disappear, and it's usually done by taking a
reference on the socket, then releasing it from the skb
destructor.

This patch is a follow-up to commit c34a7612
("net: skb_orphan() changes") and will be reverted after
catching all possible offenders if any.
Suggested-by: NDavid Miller <davem@davemloft.net>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1809,6 +1809,8 @@ static inline void skb_orphan(struct sk_buff *skb)
 		skb->destructor(skb);
 		skb->destructor = NULL;
 		skb->sk		= NULL;
+	} else {
+		BUG_ON(skb->sk);
 	}
 }