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/intel/ixgbevf/ixgbevf_main.c b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
index 4930c46..5e92cc2 100644
--- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
+++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
@@ -2912,10 +2912,10 @@
 	}
 
 	for (f = 0; f < nr_frags; f++) {
-		struct skb_frag_struct *frag;
+		const struct skb_frag_struct *frag;
 
 		frag = &skb_shinfo(skb)->frags[f];
-		len = min((unsigned int)frag->size, total);
+		len = min((unsigned int)skb_frag_size(frag), total);
 		offset = 0;
 
 		while (len) {
@@ -3096,7 +3096,7 @@
 
 	count += TXD_USE_COUNT(skb_headlen(skb));
 	for (f = 0; f < skb_shinfo(skb)->nr_frags; f++)
-		count += TXD_USE_COUNT(skb_shinfo(skb)->frags[f].size);
+		count += TXD_USE_COUNT(skb_frag_size(&skb_shinfo(skb)->frags[f]));
 
 	if (ixgbevf_maybe_stop_tx(netdev, tx_ring, count)) {
 		adapter->tx_busy++;