Fix bug with zone and zone skipping and io_limit

Fix bug with zone and zone skipping and io_limit

If you do:

dd if=/dev/zero of=/dev/shm/1M bs=1M count=1
fio --bs=4k --rw=write --filename=/dev/shm/1M --name=go \
--zoneskip=4k --zonesize=4k --io_limit=2M

then fio will exit after having performed 512KB of IO, instead
of the specified IO limit. This is a similar issue as was fixed
by commit ac002339c382 with rw sequencing - if we reach the
end of the file with a zone skip, wrap to the beginning.

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