Use controller_class_path in Rails::Generators::NamedBase#route_url

[pixeltrix]

The route_url method now returns the correct path when generating a namespaced controller with a top-level model using --model-name.

Previously, when running this command:

bin/rails generate scaffold_controller Admin/Post --model-name Post

the comments above the controller action would look like:

# GET /posts
def index
  @posts = Post.all
end

afterwards, they now look like this:

# GET /admin/posts
def index
  @posts = Post.all
end

Fixes #44662.