There is just one correct answer to:
How can I defrag on this OS?
Namely: "But why?"
Now we know:
I want to defrag it coz my laptop is getting slower, I don't have a lot of files. So, I'm hoping it could solve the problem.
Not a lot of files, so why fragmentation in particular should be the cause of a slowdown? It is more like trying to take a pot shot at the issue.
On Linux, generally, defragging is an exercise in futility. There are just two scenarios when it might have an effect:
1. Massive databases that are continuously amended.
2. HD packed to the gills with stuff.
Having used Linux OSs for some 15 years now, I have never, ever had to defrag. Frankly, I do not think there is much dissenting opinion on that.