io_uring: clean poll ->private flagging

We store a req pointer in wqe->private but also take one bit to mark
double poll entries. Replace macro helpers with inline functions for
better type checking and also name the double flag.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9a61240555c64ac0b7a9b0eb59a9efeb638a35a4.1655990418.git.asml.silence@gmail.com


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