of: unittest: Remove bogus overlay mutex release from overlay_data_add()

overlay_data_add() never takes the special overlay mutex, so it must not
be released in the error patch.

Presumably the call to of_overlay_mutex_unlock() is a relic from v1 of
the patch.

Fixes: f948d6d8 ("of: overlay: avoid race condition between applying multiple overlays")
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: NFrank Rowand <frank.rowand@sony.com>
Signed-off-by: NRob Herring <robh@kernel.org>

drivers/of/unittest.c

@@ -2165,7 +2165,6 @@ static int __init overlay_data_add(int onum)
 	ret = of_overlay_apply(info->np_overlay, &info->overlay_id);
 	if (ret < 0) {
 		pr_err("of_overlay_apply() (ret=%d), %d\n", ret, onum);
-		of_overlay_mutex_unlock();
 		goto out_free_np_overlay;
 	}