zbd: Improve random zone index generation logic

zbd: Improve random zone index generation logic

Existing random zone index generation logic is dependent on the file size.
For smaller I/O sizes the random zone index always return a particular
section of open zones. As this index is used to return one of the open zones,
it was observed that after one of the max_open_zones / job_max_open_zones limit
is reached all further I/O's are redirected to only a few open zones till they
are full.

This patch modifies the random zone index genration logic so that it is uniform
across all the open zones.

It reverts part of the commit 6463db6c1
('fio: fix interaction between offset/size limited threads and
"max_open_zones"')

Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Ankit Kumar <ankit.kumar@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>