I'll add too that you don't need to credit all your ideas in general. There are three factors I can think of that play into this.
The first is the genre you're writing/idea-using within. Academic scholarship has pretty stringent standards for citation, Dave Barry's humor columns, not so much.
The second is the degree to which you depend on the idea. If some idea is at the heart of your work, you should acknowledge it gratefully (and possibly pay license fees for it, depending on the idea's status in your jurisdiction).
The third is the degree of obviousness in your genre. You don't have to cite anyone to prove that 1+1 is 2, unless your name is Peano or Russell and you're working on mathematical meta-logic. Nobody owns common knowledge.
Because karma has been floating around as an idea on moderated-discussion forums on the Internet for a long time (since the Bronze Age of the Internet, if you will), it's pretty obvious. There are also pretty low standards for citation in the webapp world.
And if you want to go further back, who's to say Slashdot didn't steal "karma" after hearing "Karma Police" one too many times, and Radiohead had "Instant Karma" in the back of their head? And John Lennon there is obviously referring to a Hindu system of beliefs that was thousands of years old and will probably outlast all of us.
My point is that there is no way to be completely original. We are all subjugated by the tyranny of profound ideas. You just have to learn to stop worrying and love the bomb.