nvme: 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)

---
 drivers/block/nvme-core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 02351e2..cb25bdd 100644
@@ -1209,7 +1209,7 @@ static void nvme_free_queues(struct nvme_dev *dev, int lowest)
 
 	for (i = dev->queue_count - 1; i >= lowest; i--) {
 		struct nvme_queue *nvmeq = raw_nvmeq(dev, i);
-		rcu_assign_pointer(dev->queues[i], NULL);
+		RCU_INIT_POINTER(dev->queues[i], NULL);
 		call_rcu(&nvmeq->r_head, nvme_free_queue);
 		dev->queue_count--;
 	}

1.9.1