xen: netback: use __CONST_RING_SIZE not __RING_SIZE

The later causes warnings with gcc 4.5+. __CONST_RING_SIZE was introduced in
667c78af to fix this but as netback wasn't upstream at the time it did not
benefit, hence:

  CC      drivers/net/xen-netback/netback.o
drivers/net/xen-netback/netback.c:110:37: warning: variably modified 'grant_copy_op' at file scope [enabled by default]
drivers/net/xen-netback/netback.c:111:30: warning: variably modified 'meta' at file scope [enabled by default]
drivers/net/xen-netback/netback.c: In function 'xen_netbk_rx_action':
drivers/net/xen-netback/netback.c:584:6: warning: variable 'irq' set but not used [-Wunused-but-set-variable]

Thanks to Witold Baryluk for pointing this out.
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Witold Baryluk <baryluk@smp.if.uj.edu.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/common.h

@@ -106,8 +106,8 @@ struct xenvif {
 	wait_queue_head_t waiting_to_free;
 };
 
-#define XEN_NETIF_TX_RING_SIZE __RING_SIZE((struct xen_netif_tx_sring *)0, PAGE_SIZE)
-#define XEN_NETIF_RX_RING_SIZE __RING_SIZE((struct xen_netif_rx_sring *)0, PAGE_SIZE)
+#define XEN_NETIF_TX_RING_SIZE __CONST_RING_SIZE(xen_netif_tx, PAGE_SIZE)
+#define XEN_NETIF_RX_RING_SIZE __CONST_RING_SIZE(xen_netif_rx, PAGE_SIZE)
 
 struct xenvif *xenvif_alloc(struct device *parent,
 			    domid_t domid,