xen-netfront: reduce gso_max_size to account for max TCP header

The maximum packet including header that can be handled by netfront / netback
wire format is 65535. Reduce gso_max_size accordingly.

Drop skb and print warning when skb->len > 65535. This can 1) save the effort
to send malformed packet to netback, 2) help spotting misconfiguration of
netfront in the future.

Signed-off-by: Wei Liu <wei.liu2@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 9dfc120..58fadca 100644
--- a/include/xen/interface/io/netif.h
+++ b/include/xen/interface/io/netif.h
@@ -47,6 +47,7 @@
 #define _XEN_NETTXF_extra_info		(3)
 #define  XEN_NETTXF_extra_info		(1U<<_XEN_NETTXF_extra_info)
 
+#define XEN_NETIF_MAX_TX_SIZE 0xFFFF
 struct xen_netif_tx_request {
     grant_ref_t gref;      /* Reference to buffer page */
     uint16_t offset;       /* Offset within buffer page */