[RFC V2] iommu: correct group reference count

[Peng Fan <van.freenix@gmail.com>]

The basic flow for iommu_group_for_dev is:
iommu_group_get_for_dev
        |->  iommu_group_get   : increase reference count by 1.
             return group;
        |->  ops->device_group : Init/increase reference count to/by 1.
             iommu_group_add_device  : Increase reference count by 1.
             return group;

We can see that ops->device_group and iommu_group_add_device will together
increase the iommu group reference count by 2. Actually we only need 1,
but not 2. So we need add iommu_group_put after iommu_group_add_device
to make sure iommu_group_get_for_dev only increase reference count by 1.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Will Deacon <will.deacon@arm.com>
---

V1 thread: https://lkml.org/lkml/2015/11/3/304
Changes V2:
 I did not see the update about device_group when I worked out V1. So
 redo the patch and refine commit msg and rebased to latest linus'
 linux master tree.

 drivers/iommu/iommu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index abae363..9c1971b 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -852,10 +852,10 @@ struct iommu_group *iommu_group_get_for_dev(struct device *dev)
 	}
 
 	ret = iommu_group_add_device(group, dev);
-	if (ret) {
-		iommu_group_put(group);
+	iommu_group_put(group);
+
+	if (ret)
 		return ERR_PTR(ret);
-	}
 
 	return group;
 }
-- 
1.8.4