gre: Call gso_make_checksum

Call gso_make_checksum. This should have the benefit of using a
checksum that may have been previously computed for the packet.

This also adds NETIF_F_GSO_GRE_CSUM to differentiate devices that
offload GRE GSO with and without the GRE checksum offloaed.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/gre.h b/include/net/gre.h
index 70046a0..b531820 100644
--- a/include/net/gre.h
+++ b/include/net/gre.h
@@ -37,9 +37,10 @@
 		      int hdr_len);
 
 static inline struct sk_buff *gre_handle_offloads(struct sk_buff *skb,
-						  bool gre_csum)
+						  bool csum)
 {
-	return iptunnel_handle_offloads(skb, gre_csum, SKB_GSO_GRE);
+	return iptunnel_handle_offloads(skb, csum,
+					csum ? SKB_GSO_GRE_CSUM : SKB_GSO_GRE);
 }