io_uring: ensure double poll additions work with both request types

The double poll additions were centered around doing POLL_ADD on file
descriptors that use more than one waitqueue (typically one for read,
one for write) when being polled. However, it can also end up being
triggered for when we use poll triggered retry. For that case, we cannot
safely use req->io, as that could be used by the request type itself.

Add a second io_poll_iocb pointer in the structure we allocate for poll
based retry, and ensure we use the right one from the two paths.

Fixes: 18bceab1

 ("io_uring: allow POLL_ADD with double poll_wait() users")
Signed-off-by: Jens Axboe <axboe@kernel.dk>