ovl: honor flag MS_SILENT at mount

This patch hides error about missing lowerdir if MS_SILENT is set.

We use mount(NULL, "/", "overlay", MS_SILENT, NULL) for testing support of
overlayfs: syscall returns -ENODEV if it's not supported. Otherwise kernel
automatically loads module and returns -EINVAL because lowerdir is missing.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/super.c

@@ -936,7 +936,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 
 	err = -EINVAL;
 	if (!ufs->config.lowerdir) {
-		pr_err("overlayfs: missing 'lowerdir'\n");
+		if (!silent)
+			pr_err("overlayfs: missing 'lowerdir'\n");
 		goto out_free_config;
 	}