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/core/pktgen.c b/net/core/pktgen.c
index 796044a..38d6577 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -2606,13 +2606,13 @@
 			skb_shinfo(skb)->frags[i].page_offset = 0;
 			/*last fragment, fill rest of data*/
 			if (i == (frags - 1))
-				skb_shinfo(skb)->frags[i].size =
-				    (datalen < PAGE_SIZE ? datalen : PAGE_SIZE);
+				skb_frag_size_set(&skb_shinfo(skb)->frags[i],
+				    (datalen < PAGE_SIZE ? datalen : PAGE_SIZE));
 			else
-				skb_shinfo(skb)->frags[i].size = frag_len;
-			datalen -= skb_shinfo(skb)->frags[i].size;
-			skb->len += skb_shinfo(skb)->frags[i].size;
-			skb->data_len += skb_shinfo(skb)->frags[i].size;
+				skb_frag_size_set(&skb_shinfo(skb)->frags[i], frag_len);
+			datalen -= skb_frag_size(&skb_shinfo(skb)->frags[i]);
+			skb->len += skb_frag_size(&skb_shinfo(skb)->frags[i]);
+			skb->data_len += skb_frag_size(&skb_shinfo(skb)->frags[i]);
 			i++;
 			skb_shinfo(skb)->nr_frags = i;
 		}