iommu/arm-smmu: Fix group refcounting

When arm_smmu_device_group() finds an existing group due to Stream ID
aliasing, it should be taking an additional reference on that group.
Otherwise, the caller of iommu_group_get_for_dev() will inadvertently
remove the reference taken by iommu_group_add_device(), and the group
will be freed prematurely if any device is removed.
Reported-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/arm-smmu.c

@@ -1466,7 +1466,7 @@ static struct iommu_group *arm_smmu_device_group(struct device *dev)
 	}
 
 	if (group)
-		return group;
+		return iommu_group_ref_get(group);
 
 	if (dev_is_pci(dev))
 		group = pci_device_group(dev);