netvsc: reduce maximum GSO size

[ Upstream commit a50af86d ]

Hyper-V (and Azure) support using NVGRE which requires some extra space
for encapsulation headers. Because of this the largest allowed TSO
packet is reduced.

For older releases, hard code a fixed reduced value.  For next release,
there is a better solution which uses result of host offload
negotiation.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/hyperv/netvsc_drv.c

@@ -40,6 +40,8 @@
 
 #include "hyperv_net.h"
 
+/* Restrict GSO size to account for NVGRE */
+#define NETVSC_GSO_MAX_SIZE	62768
 
 #define RING_SIZE_MIN 64
 static int ring_size = 128;
@@ -852,6 +854,7 @@ static int netvsc_set_channels(struct net_device *net,
 		}
 		goto recover;
 	}
+	netif_set_gso_max_size(net, NETVSC_GSO_MAX_SIZE);
 
  out:
 	netvsc_open(net);