OPENAFS-SA-2018-002 butc: prevent TC_ReadLabel information leak

OPENAFS-SA-2018-002 butc: prevent TC_ReadLabel information leak

TC_ReadLabel (backup readlabel) does not initialize its output buffer
completely.  It leaks butc memory contents over the wire:

struct tc_tapeLabel
- up to 32 bytes from member afsname (TC_MAXTAPELEN 32)
- up to 32 bytes from member pname (TC_MAXTAPELEN 32)

Initialize the buffer.

[kaduk@mit.edu: move initialization to the RPC stub]

Change-Id: I30f4aa32801791913b397a58c36c86c019dc51ef