Add sample zipf distribution randomizer

Add sample zipf distribution randomizer

Instead of just doing purely random IO where each block is
touched exactly (or close to, depending on random map) once,
add a zipf distribution scheme where a selectable theta
defines the spread and frequency of blocks read/written.

Committing this so I don't lose it. Needs a few changes,
for instance we need to hash the zipf output so that
the spread doesn't always just favor the lower LBA range.

Signed-off-by: Jens Axboe <axboe@kernel.dk>