usb: gadget: f_fs: Stop ffs_closed NULL pointer dereference

The struct ffs_data::private_data has a pointer to
ffs_dev stored in it during the ffs_fs_mount() function
however it is not cleared when the ffs_dev is freed
later which causes the ffs_closed function to crash
with "Unable to handle kernel NULL pointer dereference"
error when using the data in ffs_data::private_data.

This clears this pointer during the ffs_free_dev clean
up function.
Signed-off-by: Jim Baxter <jim_baxter@mentor.com>
Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Harish Jenny K N <harish_kandiga@mentor.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_fs.c

@@ -3502,6 +3502,11 @@ static void _ffs_free_dev(struct ffs_dev *dev)
 	list_del(&dev->entry);
 	if (dev->name_allocated)
 		kfree(dev->name);
+
+	/* Clear the private_data pointer to stop incorrect dev access */
+	if (dev->ffs_data)
+		dev->ffs_data->private_data = NULL;
+
 	kfree(dev);
 	if (list_empty(&ffs_devices))
 		functionfs_cleanup();