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/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index 8075d11..09b8e88 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -6545,9 +6545,9 @@
 
 		frag = &skb_shinfo(skb)->frags[f];
 #ifdef IXGBE_FCOE
-		size = min_t(unsigned int, data_len, frag->size);
+		size = min_t(unsigned int, data_len, skb_frag_size(frag));
 #else
-		size = frag->size;
+		size = skb_frag_size(frag);
 #endif
 		data_len -= size;
 		f++;