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/realtek/8139cp.c b/drivers/net/ethernet/realtek/8139cp.c
index 5dcd5be..ee5da92 100644
--- a/drivers/net/ethernet/realtek/8139cp.c
+++ b/drivers/net/ethernet/realtek/8139cp.c
@@ -777,12 +777,12 @@
 		entry = NEXT_TX(entry);
 
 		for (frag = 0; frag < skb_shinfo(skb)->nr_frags; frag++) {
-			skb_frag_t *this_frag = &skb_shinfo(skb)->frags[frag];
+			const skb_frag_t *this_frag = &skb_shinfo(skb)->frags[frag];
 			u32 len;
 			u32 ctrl;
 			dma_addr_t mapping;
 
-			len = this_frag->size;
+			len = skb_frag_size(this_frag);
 			mapping = dma_map_single(&cp->pdev->dev,
 						 skb_frag_address(this_frag),
 						 len, PCI_DMA_TODEVICE);