blk-cgroup: 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)

---
 block/blk-cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 069bc20..9a95b3f 100644
@@ -337,7 +337,7 @@ static void blkg_destroy(struct blkcg_gq *blkg)
 	 * will.  Hint assignment itself can race safely.
 	 */
 	if (rcu_access_pointer(blkcg->blkg_hint) == blkg)
-		rcu_assign_pointer(blkcg->blkg_hint, NULL);
+		RCU_INIT_POINTER(blkcg->blkg_hint, NULL);
 
 	/*
 	 * If root blkg is destroyed.  Just clear the pointer since root_rl

1.9.1