keys: Timestamp new keys

mainline inclusion
from mainline-5.0-rc8
commit 7c1857bd
category: bugfix
bugzilla: 10783
CVE: NA

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

Set the timestamp on new keys rather than leaving it unset.

Fixes: 31d5a79d ("KEYS: Do LRU discard in full keyrings")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.morris@microsoft.com>

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>