net: core: Use skb_is_gso() in skb_checksum_help()

No functional changes, just minor refactoring.
Signed-off-by: Yi Li <yili@winhong.com>
Link: https://lore.kernel.org/r/20201027055904.2683444-1-yili@winhong.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -3205,7 +3205,7 @@ int skb_checksum_help(struct sk_buff *skb)
 	if (skb->ip_summed == CHECKSUM_COMPLETE)
 		goto out_set_summed;
 
-	if (unlikely(skb_shinfo(skb)->gso_size)) {
+	if (unlikely(skb_is_gso(skb))) {
 		skb_warn_bad_offload(skb);
 		return -EINVAL;
 	}