libafs: avoid flushing already freed dcaches in flush volumes

When flushing volume data, skip flushing dcache entries which
are already freed.

Reviewed-on: http://gerrit.openafs.org/7539
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 82865811e49aae8b7b8ff9f5d995dc32518b9eb8)

Change-Id: I88c9940748dcf7d179fcc2327cddcf222314560d
Reviewed-on: http://gerrit.openafs.org/9389
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>