The tricky part here was an iterator that used to take a callback and
argument for that callback as parameters.  Iterator itself didn't care
what type that argument had been; it's entirely up to callback.  The
thing is, two callbacks expect (and get) char __user * while other two
expect (and also get) char __user **.

Iterator used to use void * as "opaque data"; I've switched it to
unsigned long.  Note that there was nothing that said "it's a pointer" -
use of callback that would take e.g.  int is also perfectly legitimate. 

The rest is triviali annotation.