gue: TX support for using remote checksum offload option

Add if_tunnel flag TUNNEL_ENCAP_FLAG_REMCSUM to configure
remote checksum offload on an IP tunnel. Add logic in gue_build_header
to insert remote checksum offload option.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/uapi/linux/if_tunnel.h b/include/uapi/linux/if_tunnel.h
index 280d9e0..bd3cc11 100644
--- a/include/uapi/linux/if_tunnel.h
+++ b/include/uapi/linux/if_tunnel.h
@@ -69,6 +69,7 @@
 
 #define TUNNEL_ENCAP_FLAG_CSUM		(1<<0)
 #define TUNNEL_ENCAP_FLAG_CSUM6		(1<<1)
+#define TUNNEL_ENCAP_FLAG_REMCSUM	(1<<2)
 
 /* SIT-mode i_flags */
 #define	SIT_ISATAP	0x0001