USB: core: no need to save usb_devices_root

There is no need to save the usb_devices debugfs file as we only need it
when removing it, so have the debugfs code look it up when it is needed
instead, saving the storage.

Link: https://lore.kernel.org/r/YCubCA/trHAF7PtF@kroah.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/usb.c

@@ -950,17 +950,15 @@ static struct notifier_block usb_bus_nb = {
 	.notifier_call = usb_bus_notify,
 };
 
-static struct dentry *usb_devices_root;
-
 static void usb_debugfs_init(void)
 {
-	usb_devices_root = debugfs_create_file("devices", 0444, usb_debug_root,
-					       NULL, &usbfs_devices_fops);
+	debugfs_create_file("devices", 0444, usb_debug_root, NULL,
+			    &usbfs_devices_fops);
 }
 
 static void usb_debugfs_cleanup(void)
 {
-	debugfs_remove(usb_devices_root);
+	debugfs_remove(debugfs_lookup("devices", usb_debug_root));
 }
 
 /*