[NET] gso: Add skb_is_gso

This patch adds the wrapper function skb_is_gso which can be used instead
of directly testing skb_shinfo(skb)->gso_size.  This makes things a little
nicer and allows us to change the primary key for indicating whether an skb
is GSO (if we ever want to do that).

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/xfrm6_output.c b/net/ipv6/xfrm6_output.c
index 48fccb1..0eea60e 100644
--- a/net/ipv6/xfrm6_output.c
+++ b/net/ipv6/xfrm6_output.c
@@ -122,7 +122,7 @@
 {
 	struct sk_buff *segs;
 
-	if (!skb_shinfo(skb)->gso_size)
+	if (!skb_is_gso(skb))
 		return xfrm6_output_finish2(skb);
 
 	skb->protocol = htons(ETH_P_IP);