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/sun/niu.c b/drivers/net/ethernet/sun/niu.c
index 23740e8..73c7081 100644
--- a/drivers/net/ethernet/sun/niu.c
+++ b/drivers/net/ethernet/sun/niu.c
@@ -3594,7 +3594,7 @@
 		tb = &rp->tx_buffs[idx];
 		BUG_ON(tb->skb != NULL);
 		np->ops->unmap_page(np->device, tb->mapping,
-				    skb_shinfo(skb)->frags[i].size,
+				    skb_frag_size(&skb_shinfo(skb)->frags[i]),
 				    DMA_TO_DEVICE);
 		idx = NEXT_TX(rp, idx);
 	}
@@ -6727,9 +6727,9 @@
 	}
 
 	for (i = 0; i <  skb_shinfo(skb)->nr_frags; i++) {
-		skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
+		const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
-		len = frag->size;
+		len = skb_frag_size(frag);
 		mapping = np->ops->map_page(np->device, skb_frag_page(frag),
 					    frag->page_offset, len,
 					    DMA_TO_DEVICE);