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/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index dde6b57..ed96c54 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1094,14 +1094,16 @@
 	eat = len;
 	k = 0;
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		if (skb_shinfo(skb)->frags[i].size <= eat) {
+		int size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
+
+		if (size <= eat) {
 			skb_frag_unref(skb, i);
-			eat -= skb_shinfo(skb)->frags[i].size;
+			eat -= size;
 		} else {
 			skb_shinfo(skb)->frags[k] = skb_shinfo(skb)->frags[i];
 			if (eat) {
 				skb_shinfo(skb)->frags[k].page_offset += eat;
-				skb_shinfo(skb)->frags[k].size -= eat;
+				skb_frag_size_sub(&skb_shinfo(skb)->frags[k], eat);
 				eat = 0;
 			}
 			k++;