percpu acts like a T* const, so a const percpu should (also) be like a
T* const, *not* like a const T*, which is what the code previously did
(remember, C types are inside-out!  except when they aren't).  This,
for example, makes it sane to pass a const percpu reference.

This also makes percpu non-copyable and non-assignable and makes stuff
private.