iommu: Use iommu_group_ref_get/put() for dev->iommu_group

No reason to open code this, use the proper helper functions.
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/2-v3-328044aa278c+45e49-iommu_probe_jgg@nvidia.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu.c

@@ -500,7 +500,7 @@ static void __iommu_group_release_device(struct iommu_group *group,
 	kfree(grp_dev->name);
 	kfree(grp_dev);
 	dev->iommu_group = NULL;
-	kobject_put(group->devices_kobj);
+	iommu_group_put(group);
 }
 
 static void iommu_release_device(struct device *dev)
@@ -1067,8 +1067,7 @@ int iommu_group_add_device(struct iommu_group *group, struct device *dev)
 		goto err_free_name;
 	}
 
-	kobject_get(group->devices_kobj);
-
+	iommu_group_ref_get(group);
 	dev->iommu_group = group;
 
 	mutex_lock(&group->mutex);