block: clear iocb->private in blkdev_bio_end_io_async()

iocb_bio_iopoll() expects iocb->private to be cleared before
releasing the bio.

We already do this in blkdev_bio_end_io(), but we forgot in the
recently added blkdev_bio_end_io_async().

Fixes: 54a88eb ("block: add single bio async direct IO helper")
Cc: asml.silence@gmail.com
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220211090136.44471-1-sgarzare@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: stefano-garzarella

block/fops.c

@@ -289,6 +289,8 @@ static void blkdev_bio_end_io_async(struct bio *bio)
 	struct kiocb *iocb = dio->iocb;
 	ssize_t ret;
 
+	WRITE_ONCE(iocb->private, NULL);
+
 	if (likely(!bio->bi_status)) {
 		ret = dio->size;
 		iocb->ki_pos += ret;