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)

Change-Id: Ib9f8b45111a83178372409c717c30ccb6a1044f4
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>