[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/ipv4/ip_output.c b/net/ipv4/ip_output.c
index ca0e714..7c9f9a6 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -209,7 +209,7 @@
 		return dst_output(skb);
 	}
 #endif
-	if (skb->len > dst_mtu(skb->dst) && !skb_shinfo(skb)->gso_size)
+	if (skb->len > dst_mtu(skb->dst) && !skb_is_gso(skb))
 		return ip_fragment(skb, ip_finish_output2);
 	else
 		return ip_finish_output2(skb);
@@ -1095,7 +1095,7 @@
 	while (size > 0) {
 		int i;
 
-		if (skb_shinfo(skb)->gso_size)
+		if (skb_is_gso(skb))
 			len = size;
 		else {