staging: media: tegra-video: fix device_node use after free

At probe time this code path is followed:

 * tegra_csi_init
   * tegra_csi_channels_alloc
     * for_each_child_of_node(node, channel) -- iterates over channels
       * automatically gets 'channel'
         * tegra_csi_channel_alloc()
           * saves into chan->of_node a pointer to the channel OF node
       * automatically gets and puts 'channel'
       * now the node saved in chan->of_node has refcount 0, can disappear
   * tegra_csi_channels_init
     * iterates over channels
       * tegra_csi_channel_init -- uses chan->of_node

After that, chan->of_node keeps storing the node until the device is
removed.

of_node_get() the node and of_node_put() it during teardown to avoid any
risk.

Fixes: 1ebaeb09 ("media: tegra-video: Add support for external sensor capture")
Cc: stable@vger.kernel.org
Cc: Sowjanya Komatineni <skomatineni@nvidia.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/staging/media/tegra-video/csi.c

@@ -433,7 +433,7 @@ static int tegra_csi_channel_alloc(struct tegra_csi *csi,
 	for (i = 0; i < chan->numgangports; i++)
 		chan->csi_port_nums[i] = port_num + i * CSI_PORTS_PER_BRICK;
 
-	chan->of_node = node;
+	chan->of_node = of_node_get(node);
 	chan->numpads = num_pads;
 	if (num_pads & 0x2) {
 		chan->pads[0].flags = MEDIA_PAD_FL_SINK;
@@ -641,6 +641,7 @@ static void tegra_csi_channels_cleanup(struct tegra_csi *csi)
 			media_entity_cleanup(&subdev->entity);
 		}
 
+		of_node_put(chan->of_node);
 		list_del(&chan->list);
 		kfree(chan);
 	}