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: 3e08773 ("block: switch polling to be bio based")
Signed-off-by: Joseph Ravichandran <jravi@mit.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3783,6 +3783,7 @@ static int io_rw_init_file(struct io_kiocb *req, fmode_t mode)
 		if (!(kiocb->ki_flags & IOCB_DIRECT) || !file->f_op->iopoll)
 			return -EOPNOTSUPP;
 
+		kiocb->private = NULL;
 		kiocb->ki_flags |= IOCB_HIPRI | IOCB_ALLOC_CACHE;
 		kiocb->ki_complete = io_complete_rw_iopoll;
 		req->iopoll_completed = 0;