viced: Restrict RXAFS_FlushCPS to administrators

RXAFS_FlushCPS currently can be run by anyone, including
unauthenticated users. Forcing CPS calculation can be a relatively
resource-intensive operation, though, if done frequently enough, and
only should need to be done by administrators. Thus, only let
administrators use it.

Reviewed-on: http://gerrit.openafs.org/7572
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 568adf7d18eb17a42caa263aabc92a686f0ae121)

Change-Id: I715e7ede7ea92be65a134116ecb4d1b7e2ccd264
Reviewed-on: http://gerrit.openafs.org/9485
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>