net: Add skb_queue_is_last().

Several bits of code want to know "is this the last SKB in
a queue", and all of them implement this by hand.

Provide an common interface to make this check.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -472,6 +472,19 @@ static inline int skb_queue_empty(const struct sk_buff_head *list)
 	return list->next == (struct sk_buff *)list;
 }
 
+/**
+ *	skb_queue_is_last - check if skb is the last entry in the queue
+ *	@list: queue head
+ *	@skb: buffer
+ *
+ *	Returns true if @skb is the last buffer on the list.
+ */
+static inline bool skb_queue_is_last(const struct sk_buff_head *list,
+				     const struct sk_buff *skb)
+{
+	return (skb->next == (struct sk_buff *) list);
+}
+
 /**
  *	skb_get - reference buffer
  *	@skb: buffer to reference