rbd: fix bench-write infinite loop

I/O was continously submitted as long as there were few enough ops in
flight. If the number of 'threads' was high, or caching was turned on,
there would never be that many ops in flight, so the loop would continue
indefinitely. Instead, submit at most io_threads ops per offset.

Fixes: #3413
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage.weil@inktank.com>