hvc_xen: NULL dereference on allocation failure

commit 201a52be upstream.

If kzalloc() returns a NULL here, we pass a NULL to
xencons_disconnect_backend() which will cause an Oops.

Also I removed the __GFP_ZERO while I was at it since kzalloc() implies
__GFP_ZERO.
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/hvc/hvc_xen.c

@@ -430,9 +430,9 @@ static int __devinit xencons_probe(struct xenbus_device *dev,
 	if (devid == 0)
 		return -ENODEV;
 
-	info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL | __GFP_ZERO);
+	info = kzalloc(sizeof(struct xencons_info), GFP_KERNEL);
 	if (!info)
-		goto error_nomem;
+		return -ENOMEM;
 	dev_set_drvdata(&dev->dev, info);
 	info->xbdev = dev;
 	info->vtermno = xenbus_devid_to_vtermno(devid);