io_uring: enable poll retry for any file with ->read_iter / ->write_iter

We can have files like eventfd where it's perfectly fine to do poll
based retry on them, right now io_file_supports_async() doesn't take
that into account.

Pass in data direction and check the f_op instead of just always needing
an async worker.

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