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: NChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211012111226.760968-5-hch@lst.deSigned-off-by: NJens Axboe <axboe@kernel.dk>