Log shutdown progress

Shutting down fileservers with thousands of volumes can take a while and
it is helpful for operations to actually see that there is progress when
detaching volumes. This patch adds a log message to the fileserver log
every time 100 volumes have been detached.

Change-Id: I1685aa62335b223cf7cd3286188781318084c22f
Reviewed-on: http://gerrit.openafs.org/10797
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>