io_uring: fix a layering violation in io_iopoll_req_issued

syscall-level code can't just poke into the details of the poll cookie,
which is private information of the block layer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211012111226.760968-5-hch@lst.de


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