afs: Do not QueueVCB before osi_dnlc_purge*

afs: Do not QueueVCB before osi_dnlc_purge*

In afs_FlushVCache, when we QueueVCB, we might drop the afs_xvcache
lock (as of 76158df491f47de56d1febe1d1d2d17d316c9a74). The vcache may
still be on the DNLC, so a lookup while xvcache is dropped can cause
someone else to grab a reference to the vcache while it is being
flushed. This can cause panics and failed assertions, since someone
will have a reference to the flushed vcache, which is effectively
freed and many of the structure fields are no longer valid.

So instead, do not call QueueVCB until we have purged the vcache from
the DNLC.

Reviewed-on: http://gerrit.openafs.org/7430
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 0e81db38b549d79a3af5fce0f9f8fa48d9e6efeb)

Change-Id: Idabad9076c6c36bede3faf459abb940aae5a975c
Reviewed-on: http://gerrit.openafs.org/7502
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>