LinuxKPI: Add ktime_get_raw_fast_ns() to linux/ktime.h

It is implemented like ktime_get_raw_ns() function but uses less
precise getnanouptime() call.

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42804

(cherry picked from commit e964da141523e1eec2322082936cdb20ba0cf4a7)