[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/loopback.c b/drivers/net/loopback.c
index 43fef7d..997cbce 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -139,7 +139,7 @@
 #endif
 
 #ifdef LOOPBACK_TSO
-	if (skb_shinfo(skb)->gso_size) {
+	if (skb_is_gso(skb)) {
 		BUG_ON(skb->protocol != htons(ETH_P_IP));
 		BUG_ON(skb->nh.iph->protocol != IPPROTO_TCP);