ovl: pass string to ovl_parse_layer()

So it can be used for parsing the Opt_lowerdir.
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Link: https://lore.kernel.org/r/20240705011510.794025-2-chengzhihao1@huawei.comSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/overlayfs/params.c

@@ -365,10 +365,9 @@ static void ovl_add_layer(struct fs_context *fc, enum ovl_opt layer,
 	}
 }
 
-static int ovl_parse_layer(struct fs_context *fc, struct fs_parameter *param,
-			   enum ovl_opt layer)
+static int ovl_parse_layer(struct fs_context *fc, const char *layer_name, enum ovl_opt layer)
 {
-	char *name = kstrdup(param->string, GFP_KERNEL);
+	char *name = kstrdup(layer_name, GFP_KERNEL);
 	bool upper = (layer == Opt_upperdir || layer == Opt_workdir);
 	struct path path;
 	int err;
@@ -582,7 +581,7 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	case Opt_datadir_add:
 	case Opt_upperdir:
 	case Opt_workdir:
-		err = ovl_parse_layer(fc, param, opt);
+		err = ovl_parse_layer(fc, param->string, opt);
 		break;
 	case Opt_default_permissions:
 		config->default_permissions = true;