io-wq: fix lack of acct->nr_workers < acct->max_workers judgement

There should be this judgement before we create an io-worker

Fixes: 685fe7fe

 ("io-wq: eliminate the need for a manager thread")
Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>