net: add skb frag size accessors

To ease skb->truesize sanitization, its better to be able to localize
all references to skb frags size.

Define accessors : skb_frag_size() to fetch frag size, and
skb_frag_size_{set|add|sub}() to manipulate it.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/appletalk/ddp.c b/net/appletalk/ddp.c
index b1fe7c3..bfa9ab9 100644
--- a/net/appletalk/ddp.c
+++ b/net/appletalk/ddp.c
@@ -951,13 +951,12 @@
 	/* checksum stuff in frags */
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
 		int end;
-
+		const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 		WARN_ON(start > offset + len);
 
-		end = start + skb_shinfo(skb)->frags[i].size;
+		end = start + skb_frag_size(frag);
 		if ((copy = end - offset) > 0) {
 			u8 *vaddr;
-			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
 			if (copy > len)
 				copy = len;