futex: sched: fix UAF when free futex_exit_mutex in free_task()

hulk inclusion
category: bugfix
bugzilla: NA
CVE: NA

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If free_task() is called on error path, it will free
futex_exit_mutex of parent process and cause UAF, so
move free of futex_exit_mutex to __put_task_struct().

Fixes: f9a5a3dea71b ("futex: sched: fix kabi broken in task_struct")
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>