OPENAFS-SA-2018-002 volser: prevent AFSVolPartitionInfo(64) information leak

OPENAFS-SA-2018-002 volser: prevent AFSVolPartitionInfo(64) information leak

AFSVolPartitionInfo and AFSVolPartitionInfo64 (vos partinfo) do not
properly initialize their reply buffers.  This leaks the contents of
volserver memory over the wire:

AFSVolPartitionInfo (struct diskPartition)
- up to 24 bytes in member name (32-'/vicepa\0'))
- up to 12 bytes in member devName (32-'/vicepa/Lock/vicepa\0'))

AFSVolPartitionInfo64 (struct diskPartition64)
- up to 248 bytes in member name (256-'/vicepa\0'))
- up to 236 bytes in member devName (256-'/vicepa/Lock/vicepa\0')

Initialize the output buffers.

[kaduk@mit.edu: move memset to top-level function scope of RPC handlers]

Change-Id: If64c02f36f10f52bfbab4b21ad1f60032c223c82