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)
Signed-off-by: NAndreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: NMatthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

drivers/block/nvme-core.c

@@ -1230,7 +1230,7 @@ static void nvme_free_queues(struct nvme_dev *dev, int lowest)
 
 	for (i = dev->queue_count - 1; i >= lowest; i--) {
 		nvmeq = raw_nvmeq(dev, i);
-		rcu_assign_pointer(dev->queues[i], NULL);
+		RCU_INIT_POINTER(dev->queues[i], NULL);
 		llist_add(&nvmeq->node, &q_list);
 		dev->queue_count--;
 	}