null_blk: Improve nullb_device_##NAME##_store() readability

Introduce a local variable to make the code easier to read. This patch
does not change any functionality but makes the next patch in this
series easier to read.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: bvanassche

drivers/block/null_blk_main.c

@@ -274,10 +274,11 @@ static ssize_t									\
 nullb_device_##NAME##_store(struct config_item *item, const char *page,		\
 			    size_t count)					\
 {										\
-	if (test_bit(NULLB_DEV_FL_CONFIGURED, &to_nullb_device(item)->flags))	\
+	struct nullb_device *dev = to_nullb_device(item);			\
+										\
+	if (test_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags))			\
 		return -EBUSY;							\
-	return nullb_device_##TYPE##_attr_store(				\
-			&to_nullb_device(item)->NAME, page, count);		\
+	return nullb_device_##TYPE##_attr_store(&dev->NAME, page, count);	\
 }										\
 CONFIGFS_ATTR(nullb_device_, NAME);