timerqueue: fix kabi for struct timerqueue_head

hulk inclusion
category: bugfix
bugzilla: NA
CVE: CVE-2021-20317

---------------------------

commit d1ec2a86dbcb ("lib/timerqueue: Rely on rbtree semantics for next
timer") modifies the members of struct timerqueue_head, which breaks KABI.
This patch tries to fix it.
Signed-off-by: NXiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: NXie XiuQi <xiexiuqi@huawei.com>

include/linux/timerqueue.h

@@ -12,7 +12,12 @@ struct timerqueue_node {
 };
 
 struct timerqueue_head {
+#ifndef __GENKSYMS__
 	struct rb_root_cached rb_root;
+#else
+	struct rb_root head;
+	struct timerqueue_node *next;
+#endif
 };