aio: make use of ring code optional

aio: make use of ring code optional

Make use of the ring optional. Nobody uses the aio user mappable ring,
yet we center our completion/submission interface around it. Allow a
user to pass in INT_MAX to io_queue_init() to signal that he doesn't
care about the ring, since he uses the regular getevents() to retrieve
events. I'd like to use 0 since that more elegantly mirrors size of
the ring, but that magic value is caught and rejected by the libaio
wrapper around io_queue_init()...

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