engines/libaio: fix submit loop to use 'ld->queued' for exit

We could have head == tail for a full ring, so use the
amount queued as the gauge instead. This would be very
hard to trigger, as it would require a full ring on entry
and the first submit would have to fail to submit anything
before we loop around.

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