netif: define XEN_NETIF_MAX_TX_SIZE in public header

This is the maximum supported size of a packet. It comes from the size of
netif_tx_request.size.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>

xen/include/public/io/netif.h

@@ -65,6 +65,7 @@
 #define _NETTXF_extra_info     (3)
 #define  NETTXF_extra_info     (1U<<_NETTXF_extra_info)
 
+#define XEN_NETIF_MAX_TX_SIZE 0xFFFF
 struct netif_tx_request {
     grant_ref_t gref;      /* Reference to buffer page */
     uint16_t offset;       /* Offset within buffer page */