dax: unmap/truncate on device shutdown

Invalidate all mappings of a device-dax instance when the device is
unregistered.
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/dax/dax.c

@@ -550,6 +550,7 @@ static void unregister_dax_dev(void *dev)
 	 */
 	dax_dev->alive = false;
 	synchronize_rcu();
+	unmap_mapping_range(dax_dev->inode->i_mapping, 0, 0, 1);
 	cdev_del(cdev);
 	device_unregister(dev);
 }