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/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 8d70b44..d550895 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -334,7 +334,7 @@
 		count++;
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		unsigned long size = skb_shinfo(skb)->frags[i].size;
+		unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
 		unsigned long bytes;
 		while (size > 0) {
 			BUG_ON(copy_off > MAX_BUFFER_OFFSET);
@@ -526,7 +526,7 @@
 	for (i = 0; i < nr_frags; i++) {
 		netbk_gop_frag_copy(vif, skb, npo,
 				    skb_frag_page(&skb_shinfo(skb)->frags[i]),
-				    skb_shinfo(skb)->frags[i].size,
+				    skb_frag_size(&skb_shinfo(skb)->frags[i]),
 				    skb_shinfo(skb)->frags[i].page_offset,
 				    &head);
 	}