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/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
index f781b9a..e5246fb 100644
--- a/net/xfrm/xfrm_ipcomp.c
+++ b/net/xfrm/xfrm_ipcomp.c
@@ -90,7 +90,7 @@
 			len = dlen;
 
 		frag->page_offset = 0;
-		frag->size = len;
+		skb_frag_size_set(frag, len);
 		memcpy(skb_frag_address(frag), scratch, len);
 
 		skb->truesize += len;