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/adaptec/starfire.c b/drivers/net/ethernet/adaptec/starfire.c
index d6b0155..6d9f691 100644
--- a/drivers/net/ethernet/adaptec/starfire.c
+++ b/drivers/net/ethernet/adaptec/starfire.c
@@ -1256,12 +1256,12 @@
 			np->tx_info[entry].mapping =
 				pci_map_single(np->pci_dev, skb->data, skb_first_frag_len(skb), PCI_DMA_TODEVICE);
 		} else {
-			skb_frag_t *this_frag = &skb_shinfo(skb)->frags[i - 1];
-			status |= this_frag->size;
+			const skb_frag_t *this_frag = &skb_shinfo(skb)->frags[i - 1];
+			status |= skb_frag_size(this_frag);
 			np->tx_info[entry].mapping =
 				pci_map_single(np->pci_dev,
 					       skb_frag_address(this_frag),
-					       this_frag->size,
+					       skb_frag_size(this_frag),
 					       PCI_DMA_TODEVICE);
 		}
 
@@ -1378,7 +1378,7 @@
 					for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
 						pci_unmap_single(np->pci_dev,
 								 np->tx_info[entry].mapping,
-								 skb_shinfo(skb)->frags[i].size,
+								 skb_frag_size(&skb_shinfo(skb)->frags[i]),
 								 PCI_DMA_TODEVICE);
 						np->dirty_tx++;
 						entry++;