Potential integer overflows in kernel/arch/context.rs

Created by: philmiller-charmworks

My pull request #680 fixed one instance of potential integer overflow in checking ranges of user virtual addresses against memory mappings, in ContextZone::permission. A scan through the file shows the same issue is repeated in ContextZone::translate and Context::{permission,translate}.

This seems to suggest factoring out such sensitive code to a common range-containment testing function that the various sites can call upon.