net: Add and use skb_mark_not_on_list().

An SKB is not on a list if skb->next is NULL.

Codify this convention into a helper function and use it
where we are dequeueing an SKB and need to mark it as such.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 89283b7..c4c9e3f 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1339,6 +1339,11 @@ static inline void skb_zcopy_abort(struct sk_buff *skb)
 	}
 }
 
+static inline void skb_mark_not_on_list(struct sk_buff *skb)
+{
+	skb->next = NULL;
+}
+
 /**
  *	skb_queue_empty - check if a queue is empty
  *	@list: queue head