avoid requeueing callbacks during shutdown

avoid requeueing callbacks during shutdown

when linux calls osi_linux_free_inode_pages we can try to queue a vcb.
this is foolish as we are shutting down. so, don't.

Change-Id: I18290396d9107455453de28097e7fad58526ae06
Reviewed-on: http://gerrit.openafs.org/3297
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>