usb: gadget: f_fs: add extra check before unregister_gadget_item

ffs_closed can race with configfs_rmdir which will call config_item_release, so
add an extra check to avoid calling the unregister_gadget_item with an null
gadget item.

Signed-off-by: Rui Miguel Silva <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/function/f_fs.c

···
 static void ffs_closed(struct ffs_data *ffs)
 {
 	struct ffs_dev *ffs_obj;
+	struct f_fs_opts *opts;
 
 	ENTER();
 	ffs_dev_lock();
···
 	if (ffs_obj->ffs_closed_callback)
 		ffs_obj->ffs_closed_callback(ffs);
 
-	if (!ffs_obj->opts || ffs_obj->opts->no_configfs
-	    || !ffs_obj->opts->func_inst.group.cg_item.ci_parent)
+	if (ffs_obj->opts)
+		opts = ffs_obj->opts;
+	else
+		goto done;
+
+	if (opts->no_configfs || !opts->func_inst.group.cg_item.ci_parent
+	    || !atomic_read(&opts->func_inst.group.cg_item.ci_kref.refcount))
 		goto done;
 
 	unregister_gadget_item(ffs_obj->opts->