ovl: allow redirect_dir to default to "on"

This patch introduces a kernel config option and a module param.  Both can
be used independently to turn the default value of redirect_dir on or off.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/Kconfig

@@ -8,3 +8,17 @@ config OVERLAY_FS
 	  merged with the 'upper' object.
 
 	  For more information see Documentation/filesystems/overlayfs.txt
+
+config OVERLAY_FS_REDIRECT_DIR
+	bool "Overlayfs: turn on redirect dir feature by default"
+	depends on OVERLAY_FS
+	help
+	  If this config option is enabled then overlay filesystems will use
+	  redirects when renaming directories by default.  In this case it is
+	  still possible to turn off redirects globally with the
+	  "redirect_dir=off" module option or on a filesystem instance basis
+	  with the "redirect_dir=off" mount option.
+
+	  Note, that redirects are not backward compatible.  That is, mounting
+	  an overlay which has redirects on a kernel that doesn't support this
+	  feature will have unexpected results.

fs/overlayfs/super.c

@@ -28,6 +28,10 @@ struct ovl_dir_cache;
 
 #define OVL_MAX_STACK 500
 
+static bool ovl_redirect_dir_def = IS_ENABLED(CONFIG_OVERLAY_FS_REDIRECT_DIR);
+module_param_named(redirect_dir, ovl_redirect_dir_def, bool, 0644);
+MODULE_PARM_DESC(ovl_redirect_dir_def,
+		 "Default to on or off for the redirect_dir feature");
 
 static void ovl_dentry_release(struct dentry *dentry)
 {
@@ -702,6 +706,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	if (!ufs)
 		goto out;
 
+	ufs->config.redirect_dir = ovl_redirect_dir_def;
 	err = ovl_parse_opt((char *) data, &ufs->config);
 	if (err)
 		goto out_free_config;