volser: Check vnode length on dump

Commit aadf69eabb1962496fa93745ab560a5b48cacd61 adds length checks on
vnodes during fileserver read/write operations. Do the same thing when
we dump volume data from the volserver, to ensure that we don't
transmit incorrect data e.g. to other RO sites when releasing.

FIXES 131530

Reviewed-on: http://gerrit.openafs.org/8836
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit eb938bd18d9806f3529b7a2cbccdce5340c0e39f)

Change-Id: Ia49e7310f2e143d4eed965a5090e687be55864e4
Reviewed-on: http://gerrit.openafs.org/9505
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>