io-wq: have manager wait for all workers to exit

Instead of having to wait separately on workers and manager, just have
the manager wait on the workers. We use an atomic_t for the reference
here, as we need to start at 0 and allow increment from that. Since the
number of workers is naturally capped by the allowed nr of processes,
and that uses an int, there is no risk of overflow.

Signed-off-by: Jens Axboe <axboe@kernel.dk>