aio: add io_setup2() system call

aio: add io_setup2() system call

This is just like io_setup(), except add a flags argument to let the
caller control/define some of the io_context behavior.

Outside of the flags, we add two user pointers for future use.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>