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/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index c0ee6b6..87a6b2e 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -1106,8 +1106,8 @@
 	}
 
 	for (i = 0; i < nfrags; i++) {
-		skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
-		int len = frag->size;
+		const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
+		int len = skb_frag_size(frag);
 
 		entry = (++priv->cur_tx) % txsize;
 		desc = priv->dma_tx + entry;