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.

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>
(cherry picked from commit 845c8927ef20e245bb88bc783dc2e581b61fbaba)

Change-Id: I58d4052c69c16e4758c6accfbb2a0b1ac2aa413f
Reviewed-on: https://gerrit.openafs.org/13124
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>