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.

Change-Id: I69c9e0862972f18ecc29ff709943d9a77f2db0a9
Reviewed-on: http://gerrit.openafs.org/4641
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>