volser: ensure GCTrans transaction walk remains valid

volser: ensure GCTrans transaction walk remains valid

Commit bc56f5cc97a982ee29219e6f258b372dbfe1a020 ("volser: Delete
timed-out temporary volumes") introduced new logic to GCTrans().
Unfortunately, part of this logic temporarily drops VTRANS_LOCK in order
to call VPurgeVolume().  While this lock is dropped, other volser_trans
may be added or deleted from the allTrans list.  Therefore, GCTrans
should not trust the next pointer (nt = tt->next) which was obtained
before the lock was dropped.

One symptom observed in the field was a segfault while examining
tt->volume.  Neither tt nor volume were valid any longer, since tt had
been set from a stale nt at the top of the loop.

To repair, improve, and clarify this logic:
- Refactor so nt is assigned correctly and as late as possible.
- Add comments to explain the placement of the assigns to future
maintainers.

Reviewed-on: https://gerrit.openafs.org/13286
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 930d8ee638112ca8bf27a9528c0a527cfab54c7d)

Change-Id: I9ed2c0440b03137cb0e0ef536167be9535c90dbb
Reviewed-on: https://gerrit.openafs.org/13337
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>