xen: netback: fix printf format string warning

drivers/net/xen-netback/netback.c: In function ‘xenvif_tx_build_gops’:
drivers/net/xen-netback/netback.c:1253:8: warning: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 5 has type ‘int’ [-Wformat=]
        (txreq.offset&~PAGE_MASK) + txreq.size);
        ^

PAGE_MASK's type can vary by arch, so a cast is needed.
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
----
v2: Cast to unsigned long, since PAGE_MASK can vary by arch.
Acked-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -1250,7 +1250,7 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
 			netdev_err(queue->vif->dev,
 				   "txreq.offset: %x, size: %u, end: %lu\n",
 				   txreq.offset, txreq.size,
-				   (txreq.offset&~PAGE_MASK) + txreq.size);
+				   (unsigned long)(txreq.offset&~PAGE_MASK) + txreq.size);
 			xenvif_fatal_tx_err(queue->vif);
 			break;
 		}