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/r8169.c b/drivers/net/ethernet/realtek/r8169.c
index 2ce6070..aa39e77 100644
--- a/drivers/net/ethernet/realtek/r8169.c
+++ b/drivers/net/ethernet/realtek/r8169.c
@@ -5413,7 +5413,7 @@
 
 	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 @@
 		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))) {