vxlan: Add support for UDP checksums (v4 sending, v6 zero csums)

Added VXLAN link configuration for sending UDP checksums, and allowing
TX and RX of UDP6 checksums.

Also, call common iptunnel_handle_offloads and added GSO support for
checksums.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 622e7910..b385348 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -319,6 +319,9 @@
 	IFLA_VXLAN_PORT,	/* destination port */
 	IFLA_VXLAN_GROUP6,
 	IFLA_VXLAN_LOCAL6,
+	IFLA_VXLAN_UDP_CSUM,
+	IFLA_VXLAN_UDP_ZERO_CSUM6_TX,
+	IFLA_VXLAN_UDP_ZERO_CSUM6_RX,
 	__IFLA_VXLAN_MAX
 };
 #define IFLA_VXLAN_MAX	(__IFLA_VXLAN_MAX - 1)