Extended IDR API

[Matthew Wilcox]

I really don't like your new API.  I wish you'd discussed it before
merging it.  Here's my redesign.  Does anybody have a suggestion for
improvement?

We have a lovely new test-suite for the IDR (in tools/testing/radix-tree)
... when adding a new API, it's polite to update the test-suite too.
Do you have any plans to add test cases?

(Compile tested only; I'm at a conference.  Also, I didn't check the
kerneldoc because I don't have Sphinx installed on my laptop.)