amd-iommu: don't free dma adresses below 512MB with CONFIG_IOMMU_STRESS

This will test the automatic aperture enlargement code. This is
important because only very few devices will ever trigger this code
path. So force it under CONFIG_IOMMU_STRESS.
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

arch/x86/kernel/amd_iommu.c

@@ -824,6 +824,11 @@ static void dma_ops_free_addresses(struct dma_ops_domain *dom,
 
 	BUG_ON(i >= APERTURE_MAX_RANGES || range == NULL);
 
+#ifdef CONFIG_IOMMU_STRESS
+	if (i < 4)
+		return;
+#endif
+
 	if (address >= dom->next_address)
 		dom->need_flush = true;