io_uring: disable io-wq attaching

Moving towards making the io_wq per ring per task, so we can't really
share it between rings. Which is fine, since we've now dropped some
of that fat from it.

Retain compatibility with how attaching works, so that any attempt to
attach to an fd that doesn't exist, or isn't an io_uring fd, will fail
like it did before.

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