viced: Clear all client CPS on FlushCPS

Currently the fileserver only finds the first applicable 'client'
structure (via h_ID2Client) for a FlushCPS operation, and invalidates
the CPS for it. However, there may be many 'client' structures in
memory for the given viceid, since we may have many connections for
the same user (possibly from different hosts).

So, modify FlushCPS to find all relevant client structures, and
invalidate the CPS calculation on them.

Reviewed-on: http://gerrit.openafs.org/7574
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 387fe294f663abd9c92cd007869e2e9e8cc11d69)

Change-Id: I45b278b59649526b62eb83353cfe31465e79c712
Reviewed-on: http://gerrit.openafs.org/9487
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>