libafs: GiveUpAllCallBacks at shutdown again

Commit cee2c677d7de66a510d05978e3b41dcd5d8aca78 caused the cache
manager to give up all callbacks at shutdown. But
76158df491f47de56d1febe1d1d2d17d316c9a74 removed the call to
afs_FlushVCBs in afs_shutdown. Put it back.

Reviewed-on: http://gerrit.openafs.org/4640
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 330cc7c83bf8470e1e63f6e6d514f1e82008f167)

Change-Id: I9c2b309b482a735f5cce57aa7497b67858db2c3e
Reviewed-on: http://gerrit.openafs.org/10855
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>