io-wq: fix potential race of acct->nr_workers

Given max_worker is 1, and we currently have 1 running and it is
exiting. There may be race like:
io_wqe_enqueue worker1
no work there and timeout
unlock(wqe->lock)
->insert work
-->io_worker_exit
lock(wqe->lock)
->if(!nr_workers) //it's still 1
unlock(wqe->lock)
goto run_cancel
lock(wqe->lock)
nr_workers--
->dec_running
->worker creation fails
unlock(wqe->lock)

We enqueued one work but there is no workers, causes hung.

Signed-off-by: Hao Xu <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

fs/io-wq.c

···
 static void io_worker_exit(struct io_worker *worker)
 {
 	struct io_wqe *wqe = worker->wqe;
-	struct io_wqe_acct *acct = io_wqe_get_acct(worker);
 
 	if (refcount_dec_and_test(&worker->ref))
 		complete(&worker->ref_done);
···
 	if (worker->flags & IO_WORKER_F_FREE)
 		hlist_nulls_del_rcu(&worker->nulls_node);
 	list_del_rcu(&worker->all_list);
-	acct->nr_workers--;
 	preempt_disable();
 	io_wqe_dec_running(worker);
 	worker->flags = 0;
···
 		}
 		/* timed out, exit unless we're the last worker */
 		if (last_timeout && acct->nr_workers > 1) {
+			acct->nr_workers--;
 			raw_spin_unlock(&wqe->lock);
 			__set_current_state(TASK_RUNNING);
 			break;