LinuxKPI: add ns_to_timespec64()

Add ns_to_timespec64() which does not seem to require anything from
us given timespec64 is aliases to timespec.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D40123

(cherry picked from commit 71fde8e5070c4128e35acdb7f69a01460bce4638)