Fix time database update calculations

The time database update math assumed that the timestamps were in
nanoseconds, but at some point in the development or review process they
changed to seconds. This PR fixes the math to use seconds instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #17735