Fix bug with rw sequence offset and io_limit

Fix bug with rw sequence offset and io_limit

If you do:

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

then fio will exit prematurely after having done 128 4KB ios,
so only 512KB are read. This is because after 128 ios, we have
reache the end of the file. Handle this case and wrap around
properly.

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