Use ktime_get_ts() instead of do_gettimeofday() for latency tracking

do_gettimeofday() is not gauranteed to be monotonically increasing,
whereas the ktime_get_ts() does gaurantee it. It also has less overhead.

ktime_get_ts() is an old API, going back to 2006.
Remove some superfluous symbol exports

Symbols with names such as these should not be exported this way,
as they don't match the module's name from which they originate and
therefore clutter the global symbol namespace. Should re-think
about how to re-export them later, if needed.