Fix RpcUtils by getting method from invokerInterface if it is not null (#15477)

Author: zrlw

dubbo-rpc/dubbo-rpc-api/src/main/java/org/apache/dubbo/rpc/support/RpcUtils.java

@@ -244,9 +244,7 @@ public static boolean isOneway(URL url, Invocation inv) {
 
     private static Method getMethodByService(Invocation invocation, String service) throws NoSuchMethodException {
         Class<?> invokerInterface = invocation.getInvoker().getInterface();
-        Class<?> cls = invokerInterface != null
-                ? ReflectUtils.forName(invokerInterface.getClassLoader(), service)
-                : ReflectUtils.forName(service);
+        Class<?> cls = invokerInterface != null ? invokerInterface : ReflectUtils.forName(service);
         Method method = cls.getMethod(invocation.getMethodName(), invocation.getParameterTypes());
         if (method.getReturnType() == void.class) {
             return null;