io_uring: make IORING_POLL_ADD and IORING_POLL_REMOVE deferrable

If we defer these commands as part of a link, we have to make sure that
the SQE data has been read upfront. Integrate the poll add/remove into
the prep handling to make it safe for SQE reuse.

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