xen/fb: fix potential memory leak

This patch fixes a potential memory leak when xenfb connect to
the backend fails.

Thanks for Ian's review and comments.

[v2: reworded the commit message a bit]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Joe Jin <joe.jin@oracle.com>
Tested-by: Gurudas Pai <gurudas.pai@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Andrew Morton <akpm@linux-foundation.org>

drivers/video/xen-fbfront.c

@@ -562,26 +562,24 @@ static void xenfb_init_shared_page(struct xenfb_info *info,
 static int xenfb_connect_backend(struct xenbus_device *dev,
 				 struct xenfb_info *info)
 {
-	int ret, evtchn;
+	int ret, evtchn, irq;
 	struct xenbus_transaction xbt;
 
 	ret = xenbus_alloc_evtchn(dev, &evtchn);
 	if (ret)
 		return ret;
-	ret = bind_evtchn_to_irqhandler(evtchn, xenfb_event_handler,
+	irq = bind_evtchn_to_irqhandler(evtchn, xenfb_event_handler,
 					0, dev->devicetype, info);
-	if (ret < 0) {
+	if (irq < 0) {
 		xenbus_free_evtchn(dev, evtchn);
 		xenbus_dev_fatal(dev, ret, "bind_evtchn_to_irqhandler");
-		return ret;
+		return irq;
 	}
-	info->irq = ret;
-
  again:
 	ret = xenbus_transaction_start(&xbt);
 	if (ret) {
 		xenbus_dev_fatal(dev, ret, "starting transaction");
-		return ret;
+		goto unbind_irq;
 	}
 	ret = xenbus_printf(xbt, dev->nodename, "page-ref", "%lu",
 			    virt_to_mfn(info->page));
@@ -603,15 +601,18 @@ static int xenfb_connect_backend(struct xenbus_device *dev,
 		if (ret == -EAGAIN)
 			goto again;
 		xenbus_dev_fatal(dev, ret, "completing transaction");
-		return ret;
+		goto unbind_irq;
 	}
 
 	xenbus_switch_state(dev, XenbusStateInitialised);
+	info->irq = irq;
 	return 0;
 
  error_xenbus:
 	xenbus_transaction_end(xbt, 1);
 	xenbus_dev_fatal(dev, ret, "writing xenstore");
+ unbind_irq:
+	unbind_from_irqhandler(irq, info);
 	return ret;
 }