xen/pciback: Fix error return code in xen_pcibk_attach()

Fix to return -EFAULT from the error handling case instead of 0 when
version mismatch with pcifront.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by Jan Beulich <jbeulich@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/xen-pciback/xenbus.c

@@ -174,6 +174,7 @@ static int xen_pcibk_attach(struct xen_pcibk_device *pdev)
 				 "version mismatch (%s/%s) with pcifront - "
 				 "halting " DRV_NAME,
 				 magic, XEN_PCI_MAGIC);
+		err = -EFAULT;
 		goto out;
 	}