vol-salvage: calloc volume summary structs

vol-salvage: calloc volume summary structs

Calloc volume summary structs instead of malloc'ing them, in
vol-salvage.c. This way, new fields added to struct VolumeSummary will
be known to be zeroed by default, without needing to update all of the
allocating callers.

Reviewed-on: http://gerrit.openafs.org/4117
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit bb25fc6418bac937b31476754a09199636c24211)

Change-Id: I9533d4f841ebc64bdf49c145512471ebbb51136e
Reviewed-on: http://gerrit.openafs.org/4164
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>