Add simple way to add statistic counters to debugfs

[Andi Kleen <andi@firstfloor.org>]

This extends debugfs to make it very simple to add per cpu counters
statistics that are exported there. This is useful for all kind
of debugging situations. The counters have very little overhead
and are simpler to use than trace points.

This is the infrastructure for debugfs and a demo application 
to instrument the dcache RCU which I developed some time ago
to track down some problems there.

Posting here also so that Fengguang can use it.

Module support is still missing, but could be added without too 
much work.

-Andi