Add support for zones of random IO, with varying frequency of access

Add support for zones of random IO, with varying frequency of access

Let's say you want to ensure that 50% of the IO falls in the first 5%
of the flie, with the remaining 50% over the last 95%. Now that's
possible with random_distribution=zoned. For this particular case,
you would do:

random_distribution=zoned:50/5:50/95

Up to 64 ranges can be specified, and multiple data directions can
be given as well. The above would apply to both reads, writes, and
trims. If you wanted to have 50% of the writes fall in the first
10%, 25% in the next 10%, and the last 25% over the remaining 80%,
you could extend the above ala:

random_distribution=zoned:50/5:50/95,50/10:25/10:25/80

Signed-off-by: Jens Axboe <axboe@fb.com>