dm: use time_in_range() and time_after()

To be future-proof and for better readability the time comparisons are modified
to use time_in_range() and time_after() instead of plain, error-prone math.

Signed-off-by: Manuel Schölling <manuel.schoelling@gmx.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>