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/ethernet/broadcom/tg3.c b/drivers/net/ethernet/broadcom/tg3.c
index fe712f9..b89027c 100644
--- a/drivers/net/ethernet/broadcom/tg3.c
+++ b/drivers/net/ethernet/broadcom/tg3.c
@@ -5356,7 +5356,7 @@
 
 			pci_unmap_page(tp->pdev,
 				       dma_unmap_addr(ri, mapping),
-				       skb_shinfo(skb)->frags[i].size,
+				       skb_frag_size(&skb_shinfo(skb)->frags[i]),
 				       PCI_DMA_TODEVICE);
 
 			while (ri->fragmented) {
@@ -6510,14 +6510,14 @@
 	}
 
 	for (i = 0; i < last; i++) {
-		skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
+		const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
 		entry = NEXT_TX(entry);
 		txb = &tnapi->tx_buffers[entry];
 
 		pci_unmap_page(tnapi->tp->pdev,
 			       dma_unmap_addr(txb, mapping),
-			       frag->size, PCI_DMA_TODEVICE);
+			       skb_frag_size(frag), PCI_DMA_TODEVICE);
 
 		while (txb->fragmented) {
 			txb->fragmented = false;
@@ -6777,7 +6777,7 @@
 		for (i = 0; i <= last; i++) {
 			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
-			len = frag->size;
+			len = skb_frag_size(frag);
 			mapping = skb_frag_dma_map(&tp->pdev->dev, frag, 0,
 						   len, DMA_TO_DEVICE);