aio: utilize io_event->res2 for CQ ring

aio: utilize io_event->res2 for CQ ring

We don't use this at all, grab it to provide hints about the data.
The first hint we add is IORING_RES2_CACHEHIT, whether a read was
served out of the page cache, or if it hit media. This is useful
for buffered aio, O_DIRECT reads would never have this set (for
obvious reasons).

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