iommu/amd: Allow NULL pointer parameter for domain_flush_complete()

If domain == NULL is passed to the function, it will queue a
completion-wait command on all IOMMUs in the system.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
index 38f8a5e..0884b12 100644
--- a/drivers/iommu/amd_iommu.c
+++ b/drivers/iommu/amd_iommu.c
@@ -1177,7 +1177,7 @@
 	int i;
 
 	for (i = 0; i < amd_iommus_present; ++i) {
-		if (!domain->dev_iommu[i])
+		if (domain && !domain->dev_iommu[i])
 			continue;
 
 		/*