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>

drivers/net/vmxnet3/vmxnet3_drv.c

@@ -656,8 +656,8 @@ vmxnet3_append_frag(struct sk_buff *skb, struct Vmxnet3_RxCompDesc *rcd,
 
 	__skb_frag_set_page(frag, rbi->page);
 	frag->page_offset = 0;
-	frag->size = rcd->len;
-	skb->data_len += frag->size;
+	skb_frag_size_set(frag, rcd->len);
+	skb->data_len += rcd->len;
 	skb->truesize += PAGE_SIZE;
 	skb_shinfo(skb)->nr_frags++;
 }
@@ -745,21 +745,21 @@ vmxnet3_map_pkt(struct sk_buff *skb, struct vmxnet3_tx_ctx *ctx,
 	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-		struct skb_frag_struct *frag = &skb_shinfo(skb)->frags[i];
+		const struct skb_frag_struct *frag = &skb_shinfo(skb)->frags[i];
 
 		tbi = tq->buf_info + tq->tx_ring.next2fill;
 		tbi->map_type = VMXNET3_MAP_PAGE;
 		tbi->dma_addr = skb_frag_dma_map(&adapter->pdev->dev, frag,
-						 0, frag->size,
+						 0, skb_frag_size(frag),
 						 DMA_TO_DEVICE);
 
-		tbi->len = frag->size;
+		tbi->len = skb_frag_size(frag);
 
 		gdesc = tq->tx_ring.base + tq->tx_ring.next2fill;
 		BUG_ON(gdesc->txd.gen == tq->tx_ring.gen);
 
 		gdesc->txd.addr = cpu_to_le64(tbi->dma_addr);
-		gdesc->dword[2] = cpu_to_le32(dw2 | frag->size);
+		gdesc->dword[2] = cpu_to_le32(dw2 | skb_frag_size(frag));
 		gdesc->dword[3] = 0;
 
 		dev_dbg(&adapter->netdev->dev,