blk-ioc: 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-ioc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 1a27f45..c024a77 100644
@@ -69,7 +69,7 @@ static void ioc_destroy_icq(struct io_cq *icq)
 	 * will.  Hint assignment itself can race safely.
 	 */
 	if (rcu_access_pointer(ioc->icq_hint) == icq)
-		rcu_assign_pointer(ioc->icq_hint, NULL);
+		RCU_INIT_POINTER(ioc->icq_hint, NULL);
 
 	ioc_exit_icq(icq);
 

1.9.1