[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/drivers/net/ixgb/ixgb_main.c b/drivers/net/ixgb/ixgb_main.c
index b91e082..7eb08d9 100644
--- a/drivers/net/ixgb/ixgb_main.c
+++ b/drivers/net/ixgb/ixgb_main.c
@@ -1173,7 +1173,7 @@
 	uint16_t ipcse, tucse, mss;
 	int err;
 
-	if(likely(skb_shinfo(skb)->gso_size)) {
+	if (likely(skb_is_gso(skb))) {
 		if (skb_header_cloned(skb)) {
 			err = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
 			if (err)