xen/pvcalls-back: test for errors when calling backend_connect()

backend_connect() can fail, so switch the device to connected only if
no error occurred.

Fixes: 0a9c75c2 ("xen/pvcalls: xenbus state handling")
Cc: stable@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200511074231.19794-1-jgross@suse.comReviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/pvcalls-back.c

@@ -1088,7 +1088,8 @@ static void set_backend_state(struct xenbus_device *dev,
 		case XenbusStateInitialised:
 			switch (state) {
 			case XenbusStateConnected:
-				backend_connect(dev);
+				if (backend_connect(dev))
+					return;
 				xenbus_switch_state(dev, XenbusStateConnected);
 				break;
 			case XenbusStateClosing: