xen-pciback: fix up cleanup path when alloc fails

When allocating a pciback device fails, clear the private
field. This could lead to an use-after free, however
the 'really_probe' takes care of setting
dev_set_drvdata(dev, NULL) in its failure path (which we would
exercise if the ->probe function failed), so we we
are OK. However lets be defensive as the code can change.

Going forward we should clean up the pci_set_drvdata(dev, NULL)
in the various code-base. That will be for another day.
Reviewed-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: NJonathan Creekmore <jonathan.creekmore@gmail.com>
Signed-off-by: NDoug Goldstein <cardoe@cardoe.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xen-pciback/xenbus.c

@@ -44,7 +44,6 @@ static struct xen_pcibk_device *alloc_pdev(struct xenbus_device *xdev)
 	dev_dbg(&xdev->dev, "allocated pdev @ 0x%p\n", pdev);
 
 	pdev->xdev = xdev;
-	dev_set_drvdata(&xdev->dev, pdev);
 
 	mutex_init(&pdev->dev_lock);
 
@@ -58,6 +57,9 @@ static struct xen_pcibk_device *alloc_pdev(struct xenbus_device *xdev)
 		kfree(pdev);
 		pdev = NULL;
 	}
+
+	dev_set_drvdata(&xdev->dev, pdev);
+
 out:
 	return pdev;
 }