libafs: Flush vcaches in afs_shutdown

libafs: Flush vcaches in afs_shutdown

Currently, a few platforms (linux, linux24, solaris, irix) flush all
vcaches during shutdown. However, they do this before calling
afs_shutdown(), resulting in afs_FlushVCache queueing VCBs and
possibly trying to give the callbacks back to the server.

Instead of this, perform the flushes in afs_shutdown itself, so we do
this after we try to give up all callbacks to all servers, and we do
this while afs_shuttingdown is set, so we don't try to queue VCBs.
This also consolidates some of the duplicated code to flush all
vcaches, and now does this for all platforms.

Reviewed-on: http://gerrit.openafs.org/4641
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 80fe111f0044aa7a67215ad92210dc72cb7eb2c0)

Change-Id: I90afacbdd7ef0f52de4589001ec9bb8ffd292782
Reviewed-on: http://gerrit.openafs.org/4702
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>