libafs: avoid resetting the dynroot volume every 10 minutes

libafs: avoid resetting the dynroot volume every 10 minutes

The dynroot volumes are synthetic, so do not need to be reset every time
the background daemon checks the volumes.

The results of osi_Time() is a signed 32-bit integer, and the volume
expireTime is an signed 32-bit integer, so use signed 32-bit integers
for the expiry check.

Change-Id: Ib92157686c1d8b84a63d409cb148155705953b6d
Reviewed-on: https://gerrit.openafs.org/11895
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>