signal: fix kabi changes in struct task_struct

hulk inclusion
category: kabi
bugzilla: NA
CVE: CVE-2020-10741, CVE-2020-12826

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Commit d1e7fd64 ("signal: Extend exec_id to 64bits") can fixes
CVE-2020-10741 and CVE-2020-12826, but it introduces a kabi change
in struct task_strcut. Fix this kabi broken by using another new
64bits variables parent_exec_id_u64 and self_exec_id_u64.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>