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/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
index 91a6b71..adbda182f 100644
--- a/drivers/net/ethernet/sfc/rx.c
+++ b/drivers/net/ethernet/sfc/rx.c
@@ -481,7 +481,7 @@
 		skb_frag_set_page(skb, 0, page);
 		skb_shinfo(skb)->frags[0].page_offset =
 			efx_rx_buf_offset(efx, rx_buf);
-		skb_shinfo(skb)->frags[0].size = rx_buf->len;
+		skb_frag_size_set(&skb_shinfo(skb)->frags[0], rx_buf->len);
 		skb_shinfo(skb)->nr_frags = 1;
 
 		skb->len = rx_buf->len;