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/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 1e20b64..1c9bf8b5 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1512,13 +1512,14 @@
 				err = -EMSGSIZE;
 				goto error;
 			}
-			if (getfrag(from, skb_frag_address(frag)+frag->size,
+			if (getfrag(from,
+				    skb_frag_address(frag) + skb_frag_size(frag),
 				    offset, copy, skb->len, skb) < 0) {
 				err = -EFAULT;
 				goto error;
 			}
 			sk->sk_sndmsg_off += copy;
-			frag->size += copy;
+			skb_frag_size_add(frag, copy);
 			skb->len += copy;
 			skb->data_len += copy;
 			skb->truesize += copy;