uio_hv_generic: Fix kernel NULL pointer dereference in hv_uio_rescind

For primary VM Bus channels, primary_channel pointer is always NULL. This
pointer is valid only for the secondary channels. Also, rescind callback
is meant for primary channels only.

Fix NULL pointer dereference by retrieving the device_obj from the parent
for the primary channel.

Cc: stable@vger.kernel.org
Fixes: ca3cda6f ("uio_hv_generic: add rescind support")
Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
Link: https://lore.kernel.org/r/20240829071312.1595-2-namjain@linux.microsoft.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/uio/uio_hv_generic.c

@@ -106,10 +106,11 @@ static void hv_uio_channel_cb(void *context)
 
 /*
  * Callback from vmbus_event when channel is rescinded.
+ * It is meant for rescind of primary channels only.
  */
 static void hv_uio_rescind(struct vmbus_channel *channel)
 {
-	struct hv_device *hv_dev = channel->primary_channel->device_obj;
+	struct hv_device *hv_dev = channel->device_obj;
 	struct hv_uio_private_data *pdata = hv_get_drvdata(hv_dev);
 
 	/*