ubik: Zero header before writing to disk

The ubik disk header contains a padding field, which is never
zeroed before being written out to disk. This means that there is
a shorts worth of stack garbage in every ubik label!

Just zero the whole structure with memset before we populate it.

Caught by coverity (#986011)

Reviewed-on: http://gerrit.openafs.org/9272
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit d8fa251a14e9469fb6ec722d100b6a8557248694)

Change-Id: I1d3baf4ced6583f22458155edcc16344acc5757e
Reviewed-on: http://gerrit.openafs.org/9357
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>