xen-netback: handle IPv6 TCP GSO packets from the guest

This patch adds a xenstore feature flag, festure-gso-tcpv6, to advertise
that netback can handle IPv6 TCP GSO packets. It creates SKB_GSO_TCPV6 skbs
if the frontend passes an extra segment with the new type
XEN_NETIF_GSO_TYPE_TCPV6 added to netif.h.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/xen/interface/io/netif.h b/include/xen/interface/io/netif.h
index c9e8184..5e766eb 100644
--- a/include/xen/interface/io/netif.h
+++ b/include/xen/interface/io/netif.h
@@ -58,6 +58,13 @@
  */
 
 /*
+ * "feature-gso-tcpv4" and "feature-gso-tcpv6" advertise the capability to
+ * handle large TCP packets (in IPv4 or IPv6 form respectively). Neither
+ * frontends nor backends are assumed to be capable unless the flags are
+ * present.
+ */
+
+/*
  * This is the 'wire' format for packets:
  *  Request 1: xen_netif_tx_request  -- XEN_NETTXF_* (any flags)
  * [Request 2: xen_netif_extra_info]    (only if request 1 has XEN_NETTXF_extra_info)
@@ -102,8 +109,9 @@
 #define _XEN_NETIF_EXTRA_FLAG_MORE	(0)
 #define  XEN_NETIF_EXTRA_FLAG_MORE	(1U<<_XEN_NETIF_EXTRA_FLAG_MORE)
 
-/* GSO types - only TCPv4 currently supported. */
+/* GSO types */
 #define XEN_NETIF_GSO_TYPE_TCPV4	(1)
+#define XEN_NETIF_GSO_TYPE_TCPV6	(2)
 
 /*
  * This structure needs to fit within both netif_tx_request and