backend: terminate loop if we didn't do IO

The below commit modified how we clear stats, and that can sometimes
cause fio to hang around when ramp + runtime has been exceeded. The
below job file showed the issue:

[test-job]
bs=4k
rw=read
direct=1
runtime=6s
ramp_time=5s
ioengine=null
iodepth=32
size=100g

Where it would do the ramp time, then count down the 6s of runtime, then
fio would just sit there busy looping.

Fixes: ac28d9053679 ("Fixup bw/iops logging for short runs")
Signed-off-by: Jens Axboe <axboe@fb.com>