vol: Call memset with the correct size

Call memset with the size of the structure we're initialising to 0,
and not the size of the pointer to that structure.

Caught by clang's new warnings.

Reviewed-on: http://gerrit.openafs.org/7083
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 408afc5690fc8ac3b6942983765ab36e63d2534d)

Change-Id: I25fd235ffd7478295bf205b232ecc5816f962d3f
Reviewed-on: http://gerrit.openafs.org/9472
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>