net: Add safe reverse SKB queue walkers.

Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1801,6 +1801,15 @@ static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
 		     prefetch(skb->prev), (skb != (struct sk_buff *)(queue));	\
 		     skb = skb->prev)
 
+#define skb_queue_reverse_walk_safe(queue, skb, tmp)				\
+		for (skb = (queue)->prev, tmp = skb->prev;			\
+		     skb != (struct sk_buff *)(queue);				\
+		     skb = tmp, tmp = skb->prev)
+
+#define skb_queue_reverse_walk_from_safe(queue, skb, tmp)			\
+		for (tmp = skb->prev;						\
+		     skb != (struct sk_buff *)(queue);				\
+		     skb = tmp, tmp = skb->prev)
 
 static inline bool skb_has_frag_list(const struct sk_buff *skb)
 {