iommu: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

The use of "rcu_assign_pointer()" is NULLing out the pointer.
According to RCU_INIT_POINTER()'s block comment:
"1.   This use of RCU_INIT_POINTER() is NULLing out the pointer"
it is better to use it instead of rcu_assign_pointer() because it has a
smaller overhead.

The following Coccinelle semantic patch was used:
@@
@@

- rcu_assign_pointer
+ RCU_INIT_POINTER
  (..., NULL)
Signed-off-by: NAndreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>

drivers/iommu/dmar.c

@@ -247,7 +247,7 @@ int dmar_remove_dev_scope(struct dmar_pci_notify_info *info, u16 segment,
 
 	for_each_active_dev_scope(devices, count, index, tmp)
 		if (tmp == &info->dev->dev) {
-			rcu_assign_pointer(devices[index].dev, NULL);
+			RCU_INIT_POINTER(devices[index].dev, NULL);
 			synchronize_rcu();
 			put_device(tmp);
 			return 1;