iommu: Fix reference count leak in iommu_group_alloc.

kobject_init_and_add() takes reference even when it fails.
Thus, when kobject_init_and_add() returns an error,
kobject_put() must be called to properly clean up the kobject.

Fixes: d72e31c9 ("iommu: IOMMU Groups")
Signed-off-by: Qiushi Wu <wu000273@umn.edu>
Link: https://lore.kernel.org/r/20200527210020.6522-1-wu000273@umn.eduSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu.c

@@ -510,7 +510,7 @@ struct iommu_group *iommu_group_alloc(void)
 				   NULL, "%d", group->id);
 	if (ret) {
 		ida_simple_remove(&iommu_group_ida, group->id);
-		kfree(group);
+		kobject_put(&group->kobj);
 		return ERR_PTR(ret);
 	}