io_uring: convert poll path to use io_cmd_type

Remove struct io_poll_iocb from io_kiocb, and convert the poll path to
use the io_cmd_type approach instead.

While at it, rename io_poll_iocb to io_poll which is consistent with the
other request type private structures.

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