io-wq: allow grabbing existing io-wq

Export a helper to attach to an existing io-wq, rather than setting up
a new one. This is doable now that we have reference counted io_wq's.

Reported-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>