io_uring: consistently use the EPOLL* defines

POLL* are unannotated values for the userspace ABI, while everything
in-kernel should use EPOLL* and the __poll_t type.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220518084005.3255380-6-hch@lst.de


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