io_uring: fix uninitialized field in rw io_kiocb

io_rw_init_file does not initialize kiocb->private, so when iocb_bio_iopoll
reads kiocb->private it can contain uninitialized data.

Fixes: 3e08773c ("block: switch polling to be bio based")
Signed-off-by: Joseph Ravichandran <jravi@mit.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>