io_uring: don't use kiocb.private to store buf_index

kiocb.private is used in iomap_dio_rw() so store buf_index separately.

Signed-off-by: Bijan Mottahedeh <bijan.mottahedeh@oracle.com>

Move 'buf_index' to a hole in io_kiocb.

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