volinfo: release volume header ihandles when done

After scanning the volume, release the header ihandles
for the volume.

Reviewed-on: http://gerrit.openafs.org/5099
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 4aa72b4308bae8e1d789682e7ead83bcadded862)

Change-Id: I7a24892fc5f6d77b4aaff4a280063bc13cf4d81e
Reviewed-on: http://gerrit.openafs.org/11263
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>