OPENAFS-SA-2018-002 budb: prevent BUDB_* information leaks

OPENAFS-SA-2018-002 budb: prevent BUDB_* information leaks

The following budb RPCs do not initialize their output correctly.
This leaks buserver memory contents over the wire:

BUDB_FindLatestDump (backup dump)
BUDB_FindDump (backup volrestore, diskrestore, volsetrestore)
BUDB_GetDumps (backup dumpinfo)
BUDB_FindLastTape (backup dump)

struct budb_dumpEntry
- up to 32 bytes in member volumeSetName
- up to 256 bytes in member dumpPath
- up to 32 bytes in member name
- up to 32 bytes in member tape.tapeServer
- up to 32 bytes in member tape.format
- up to 256 bytes in member dumper.name
- up to 128 bytes in member dumper.instance
- up to 256 bytes in member dumper.cell

Initialize the buffer in common routine FillDumpEntry.

(cherry picked from commit e96771471134102d3879a0ac8b2c4ef9d91a61b8)

Change-Id: I85ec8a21966386baa8243326072e5730726cba96