virtio-rng: make device ready before making request

Current virtio-rng does a entropy request before DRIVER_OK, this
violates the spec:

virtio spec requires that all drivers set DRIVER_OK
before using devices.

Further, kernel will ignore the interrupt after commit
8b4ec69d ("virtio: harden vring IRQ").

Fixing this by making device ready before the request.

Cc: stable@vger.kernel.org
Fixes: 8b4ec69d ("virtio: harden vring IRQ")
Fixes: f7f510ec ("virtio: An entropy device, as suggested by hpa.")
Reported-and-tested-by: syzbot+5b59d6d459306a556f54@syzkaller.appspotmail.com
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220608061422.38437-1-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>

drivers/char/hw_random/virtio-rng.c

@@ -159,6 +159,8 @@ static int probe_common(struct virtio_device *vdev)
 		goto err_find;
 	}
 
+	virtio_device_ready(vdev);
+
 	/* we always have a pending entropy request */
 	request_entropy(vi);