skbuff: Add frag list abstraction interfaces.

With the hope that these can be used to eliminate direct
references to the frag list implementation.

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

include/linux/skbuff.h

@@ -1077,7 +1077,7 @@ extern void skb_add_rx_frag(struct sk_buff *skb, int i, struct page *page,
 			    int off, int size);
 
 #define SKB_PAGE_ASSERT(skb) 	BUG_ON(skb_shinfo(skb)->nr_frags)
-#define SKB_FRAG_ASSERT(skb) 	BUG_ON(skb_shinfo(skb)->frag_list)
+#define SKB_FRAG_ASSERT(skb) 	BUG_ON(skb_has_frags(skb))
 #define SKB_LINEAR_ASSERT(skb)  BUG_ON(skb_is_nonlinear(skb))
 
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
@@ -1716,6 +1716,25 @@ static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
 		     skb = skb->prev)
 
 
+static inline bool skb_has_frags(const struct sk_buff *skb)
+{
+	return skb_shinfo(skb)->frag_list != NULL;
+}
+
+static inline void skb_frag_list_init(struct sk_buff *skb)
+{
+	skb_shinfo(skb)->frag_list = NULL;
+}
+
+static inline void skb_frag_add_head(struct sk_buff *skb, struct sk_buff *frag)
+{
+	frag->next = skb_shinfo(skb)->frag_list;
+	skb_shinfo(skb)->frag_list = frag;
+}
+
+#define skb_walk_frags(skb, iter)	\
+	for (iter = skb_shinfo(skb)->frag_list; iter; iter = iter->next)
+
 extern struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned flags,
 					   int *peeked, int *err);
 extern struct sk_buff *skb_recv_datagram(struct sock *sk, unsigned flags,