ovl: port ovl_check_empty_dir() to cred guard

Use the scoped ovl cred guard.

Link: https://patch.msgid.link/[e-mail]
Reviewed-by: Amir Goldstein <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/overlayfs/readdir.c

···
 	int err;
 	struct ovl_cache_entry *p, *n;
 	struct rb_root root = RB_ROOT;
-	const struct cred *old_cred;
 
-	old_cred = ovl_override_creds(dentry->d_sb);
-	err = ovl_dir_read_merged(dentry, list, &root);
-	ovl_revert_creds(old_cred);
+	with_ovl_creds(dentry->d_sb)
+		err = ovl_dir_read_merged(dentry, list, &root);
 	if (err)
 		return err;