Anywhere really, it's just the phrase 'there's a thin [or fine] line between...' modified to say the police are that line, between lawful order & disorder.
Apparently it's political in the US, I have no idea, but as I understand it the maintainer just means 'I am here reviewing the change to keep the kernel in good order'.
The maintainer might mean that, but words have meaning. That particular phrase is overly charged and carries a specific connotation surrounding the idea that police are the sole line keeping society in shape.
It’s a poor choice of words for such (relatively) public communication.
These words have the meaning you're implying to at least some people in the USA, it was new to me.
I don't know his full biography, seems to be Chinese born and went to MIT, but he signs off 'Cheers', I think it's a reasonable possibility that he doesn't mean whatever politically charged US meaning it has by it.
You're, with your US perspective, saying 'hey words have meaning you know, don't downplay murdering homosexuals' while millions^ of people smoke fags in the UK every day.
(^probably? Maybe not any more, a lot of fag-smoking relative to murder at any rate.)
From what I saw on urbandictionary, it seems more likely to be something cops in high crime areas in the UK say.