Revert "bdi: fix kabi for struct backing_dev_info"

hulk inclusion
category: bugfix
bugzilla: 30939
CVE: NA
---------------------------

The kabi can be broken before official release.

This reverts commit f8589079659b51222d86a1cb8fd9129752b0d97c.
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-By: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

include/linux/backing-dev-defs.h

@@ -201,14 +201,10 @@ struct backing_dev_info {
 #endif
 	wait_queue_head_t wb_waitq;
 
-#ifndef __GENKSYMS__
 	union {
 		struct rcu_device *rcu_dev;
 		struct device *dev;
 	};
-#else
-	struct device *dev;
-#endif
 	struct device *owner;
 
 	struct timer_list laptop_mode_wb_timer;