Revert "posix-cpu-timers: Avoid undefined behaviour in timespec64_to_ns()"

hulk inclusion
category: bugfix
bugzilla: 185900, https://gitee.com/openeuler/kernel/issues/I4MOGB


CVE: NA

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This reverts commit 2a3c5819.
Commit c0b52741 fixes undefined behaviour in timespec64_to_ns(),
so just revert it.

Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Reviewed-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>