xen/pvcalls: use WARN_ON(1) instead of __WARN()

__WARN() is an internal helper that is only available on
some architectures, but causes a build error e.g. on ARM64
in some configurations:

drivers/xen/pvcalls-back.c: In function 'set_backend_state':
drivers/xen/pvcalls-back.c:1097:5: error: implicit declaration of function '__WARN' [-Werror=implicit-function-declaration]

Unfortunately, there is no equivalent of BUG() that takes no
arguments, but WARN_ON(1) is commonly used in other drivers
and works on all configurations.

Fixes: 7160378206b2 ("xen/pvcalls: xenbus state handling")
Signed-off-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Stefano Stabellini <[e-mail]>
Signed-off-by: Boris Ostrovsky <[e-mail]>

drivers/xen/pvcalls-back.c

···
 				xenbus_switch_state(dev, XenbusStateClosing);
 				break;
 			default:
-				__WARN();
+				WARN_ON(1);
 			}
 			break;
 		case XenbusStateInitWait:
···
 				xenbus_switch_state(dev, XenbusStateClosing);
 				break;
 			default:
-				__WARN();
+				WARN_ON(1);
 			}
 			break;
 		case XenbusStateConnected:
···
 				xenbus_switch_state(dev, XenbusStateClosing);
 				break;
 			default:
-				__WARN();
+				WARN_ON(1);
 			}
 			break;
 		case XenbusStateClosing:
···
 				xenbus_switch_state(dev, XenbusStateClosed);
 				break;
 			default:
-				__WARN();
+				WARN_ON(1);
 			}
 			break;
 		default:
-			__WARN();
+			WARN_ON(1);
 		}
 	}
 }