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/ethernet/realtek/8139cp.c

@@ -777,12 +777,12 @@ static netdev_tx_t cp_start_xmit (struct sk_buff *skb,
 		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);

drivers/net/ethernet/realtek/r8169.c

@@ -5413,7 +5413,7 @@ static int rtl8169_xmit_frags(struct rtl8169_private *tp, struct sk_buff *skb,
 
 	entry = tp->cur_tx;
 	for (cur_frag = 0; cur_frag < info->nr_frags; cur_frag++) {
-		skb_frag_t *frag = info->frags + cur_frag;
+		const skb_frag_t *frag = info->frags + cur_frag;
 		dma_addr_t mapping;
 		u32 status, len;
 		void *addr;
@@ -5421,7 +5421,7 @@ static int rtl8169_xmit_frags(struct rtl8169_private *tp, struct sk_buff *skb,
 		entry = (entry + 1) % NUM_TX_DESC;
 
 		txd = tp->TxDescArray + entry;
-		len = frag->size;
+		len = skb_frag_size(frag);
 		addr = skb_frag_address(frag);
 		mapping = dma_map_single(d, addr, len, DMA_TO_DEVICE);
 		if (unlikely(dma_mapping_error(d, mapping))) {