I'm not a lawyer, but from what I have read about entrapment if they tried to charge you for viewing porn because you clicked on the link, that would be entrapment. However, they are not doing that. They are using the fact that you clicked on the link as probable cause for searching your computer. They then charge you having the child porn on your computer that is unrelated to link that you clicked. If you didn't have any porn on your computer, then you likely would not be able to be charged for clicking on the link (since you could then claim entrapment).