ubik: remove redundant memset from udisk_write

ubik: remove redundant memset from udisk_write

When udisk_write is extending the database, DRead will return a null
buffer.  udisk_write then calls DNew to get a brand new buffer for the
extension write, and clears it with memset.  However, this is redundant,
since DNew has already cleared the new buffer.

Remove the redundant memset.

No functional change should be incurred by this commit.

Change-Id: Ia6768098fb3c67475c8948c874b92b91bf17cdb7
Reviewed-on: https://gerrit.openafs.org/12621
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>