io_uring: io_wq_create() returns an error pointer, not NULL

syzbot reported an issue where we crash at setup time if failslab is
used. The issue is that io_wq_create() returns an error pointer on
failure, not NULL. Hence io_uring thought the io-wq was setup just
fine, but in reality it's a garbage error pointer.

Use IS_ERR() instead of a NULL check, and assign ret appropriately.

Reported-by: syzbot+221cc24572a2fed23b6b@syzkaller.appspotmail.com
Fixes: 561fb04a ("io_uring: replace workqueue usage with io-wq")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3489,8 +3489,9 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 	/* Do QD, or 4 * CPUS, whatever is smallest */
 	concurrency = min(ctx->sq_entries, 4 * num_online_cpus());
 	ctx->io_wq = io_wq_create(concurrency, ctx->sqo_mm);
-	if (!ctx->io_wq) {
-		ret = -ENOMEM;
+	if (IS_ERR(ctx->io_wq)) {
+		ret = PTR_ERR(ctx->io_wq);
+		ctx->io_wq = NULL;
 		goto err;
 	}