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/tundra/tsi108_eth.c b/drivers/net/ethernet/tundra/tsi108_eth.c
index a03996c..a8df7ec 100644
--- a/drivers/net/ethernet/tundra/tsi108_eth.c
+++ b/drivers/net/ethernet/tundra/tsi108_eth.c
@@ -709,13 +709,13 @@
 			data->txring[tx].len = skb_headlen(skb);
 			misc |= TSI108_TX_SOF;
 		} else {
-			skb_frag_t *frag = &skb_shinfo(skb)->frags[i - 1];
+			const skb_frag_t *frag = &skb_shinfo(skb)->frags[i - 1];
 
 			data->txring[tx].buf0 = skb_frag_dma_map(NULL, frag,
 								 0,
-								 frag->size,
+								 skb_frag_size(frag),
 								 DMA_TO_DEVICE);
-			data->txring[tx].len = frag->size;
+			data->txring[tx].len = skb_frag_size(frag);
 		}
 
 		if (i == frags - 1)