zipf/pareto: use size= if given, not always device size

We used the full device/file size for the zipf or pareto init,
this breaks if size= is specifically given. This causes fio
to generate offsets outside the specified size= range, and fio
will then just exit.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Merge branch 'master' of ssh://brick.kernel.dk/data/git/fio