OPENAFS-SA-2016-002 VldbListByAttributes information leak

OPENAFS-SA-2016-002 VldbListByAttributes information leak

The VldbListByAttributes structure is used as an input to several
RPCs; it contains a Mask field that controls
which of the other fields will actually be read by the server
during the RPC processing.  Unfortunately, the client only
wrote to the fields indicated by the mask, leaving the other
fields uninitialized for transmission on the wire, leaking
some contents of client memory.

Plug the information leak by zeroing the entire structure before use.

FIXES 132847

Change-Id: I14964e98a57ba6ef060c6e392497f1ebd3afe042