Add support for absolute random zones

Add support for absolute random zones

We currently support random_distribution=zoned, which allows the user
to specify a percentage of access to a zoned define as a percentage
of the file/device size. This commit adds support for zoned_abs,
which works exactly like zoned, except you give the zone size in
an absolute value.

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