io_uring: add and use struct io_rw for read/writes

Put the kiocb in struct io_rw, and add the addr/len for the request as
well. Use the kiocb->private field for the buffer index for fixed reads
and writes.

Any use of kiocb->ki_filp is flipped to req->file. It's the same thing,
and less confusing.

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