iommu: Don't overwrite domain pointer when there is no default_domain

IOMMU drivers that do not support default domains, but make
use of the the group->domain pointer can get that pointer
overwritten with NULL on device add/remove.

Make sure this can't happen by only overwriting the domain
pointer when it is NULL.

Cc: stable@vger.kernel.org # v4.4+
Fixes: 1228236d ('iommu: Move default domain allocation to iommu_group_get_for_dev()')
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu.c

@@ -848,7 +848,8 @@ struct iommu_group *iommu_group_get_for_dev(struct device *dev)
 	if (!group->default_domain) {
 		group->default_domain = __iommu_domain_alloc(dev->bus,
 							     IOMMU_DOMAIN_DMA);
-		group->domain = group->default_domain;
+		if (!group->domain)
+			group->domain = group->default_domain;
 	}
 
 	ret = iommu_group_add_device(group, dev);