io_uring: make apoll_events a __poll_t

apoll_events is fed to vfs_poll and the poll tables, so it should be
a __poll_t.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220518084005.3255380-5-hch@lst.de


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