Fix crash and precision of ETA with zones

If zonesize was bigger than zoneskip, we could encounter a divide by
zero when calculating the number of bytes.

Additionally, the math was just wrong for most cases of zone settings.
Improve that.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
stat: fix wrong type used for 32-bit compiles

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Fix zones for numjobs=x, where x > 1

In the options fixup, we auto-set open_files to 1 if it hasn't been set.
So any duplicated jobs will clear the zone. Fixup the check to properly
check for > 1 open files.

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