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>

drivers/atm/eni.c

@@ -1136,7 +1136,7 @@ DPRINTK("doing direct send\n"); /* @@@ well, this doesn't work anyway */
 				put_dma(tx->index,eni_dev->dma,&j,(unsigned long)
 				    skb_frag_page(&skb_shinfo(skb)->frags[i]) +
 					skb_shinfo(skb)->frags[i].page_offset,
-				    skb_shinfo(skb)->frags[i].size);
+				    skb_frag_size(&skb_shinfo(skb)->frags[i]));
 	}
 	if (skb->len & 3)
 		put_dma(tx->index,eni_dev->dma,&j,zeroes,4-(skb->len & 3));