futex: sched: fix kabi broken in task_struct

hulk inclusion
category: bugfix
bugzilla: NA
CVE: NA

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

Fix kabi broken introduced by
c5f21df94d82 ("futex: Replace PF_EXITPIDONE with a state")
5c7385e6eb43 ("futex: Add mutex around futex exit")

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Jian Cheng <cj.chengjian@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>