Fix disk utilization for delayed jobs

Fix disk utilization for delayed jobs

We initialized it all before starting the jobs, so we would
easily see disk stats that were too low for jobs that ended
up being started later.

Fix this by moving the disk util init right before the jobs
are spawned.

Reported-by: Shrirang Phansalkar <sphansalkar@stec-inc.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>