add_filter( 'woocommerce_cart_product_subtotal', 'modify_cart_product_subtotal', 10, 4 );
function modify_cart_product_subtotal( $product_subtotal, $product, $quantity, $cart ) {
    if ( $product->get_stock_quantity() < $quantity ) {
        return $product->get_stock_quantity() * $product->get_price();
    }
    return $product_subtotal;
}