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>