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/virtio_net.c b/drivers/net/virtio_net.c
index b8225f3..0d4841b 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -147,14 +147,14 @@
 	skb_frag_t *f;
 
 	f = &skb_shinfo(skb)->frags[i];
-	f->size = min((unsigned)PAGE_SIZE - offset, *len);
+	skb_frag_size_set(f, min((unsigned)PAGE_SIZE - offset, *len));
 	f->page_offset = offset;
 	__skb_frag_set_page(f, page);
 
-	skb->data_len += f->size;
-	skb->len += f->size;
+	skb->data_len += skb_frag_size(f);
+	skb->len += skb_frag_size(f);
 	skb_shinfo(skb)->nr_frags++;
-	*len -= f->size;
+	*len -= skb_frag_size(f);
 }
 
 static struct sk_buff *page_to_skb(struct virtnet_info *vi,