io_uring/io-wq: ignore non-busy worker going to sleep

When an io-wq worker goes to sleep, it checks if there's work to do.
If there is, it'll create a new worker. But if this worker is currently
idle, it'll either get woken right back up immediately, or someone
else has already created the necessary worker to handle this work.

Only go through the worker creation logic if the current worker is
currently handling a work item. That means it's being scheduled out as
part of handling that work, not just going to sleep on its own.

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