null_blk: use available 'dev' in nullb_device_power_store()

We already have this pointer, no need to use to_nullb_device()
again.
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/null_blk.c

@@ -375,7 +375,7 @@ static ssize_t nullb_device_power_store(struct config_item *item,
 
 		set_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags);
 		dev->power = newp;
-	} else if (to_nullb_device(item)->power && !newp) {
+	} else if (dev->power && !newp) {
 		mutex_lock(&lock);
 		dev->power = newp;
 		null_del_dev(dev->nullb);