[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/include/linux/netdevice.h b/include/linux/netdevice.h
index 85f99f6..0359a64 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1001,7 +1001,7 @@
 
 static inline int skb_gso_ok(struct sk_buff *skb, int features)
 {
-	return net_gso_ok(features, skb_shinfo(skb)->gso_size ?
+	return net_gso_ok(features, skb_is_gso(skb) ?
 				    skb_shinfo(skb)->gso_type : 0);
 }