OPENAFS-SA-2024-003: xdr: Initialize memory for INOUT args

OPENAFS-SA-2024-003: xdr: Initialize memory for INOUT args

CVE-2024-10397

Currently, there are a few callers of RPCs that specify some data for
an INOUT parameter, but do not initialize the memory for that data.
This can result in the uninitialized memory being sent to the peer
when the argument is processed as an IN argument. Simply clear the
relevant data before running the RPC to avoid this.

The relevant RPCs and arguments are:

- For RMTSYS_Pioctl, the 'OutData' argument.

- For BUDB_GetVolumes, the 'volumes' argument.
-- via DBLookupByVolume -> bcdb_LookupVolume -> ubik_BUDB_GetVolumes
-- and via bc_Restorer -> bcdb_FindVolumes -> ubik_BUDB_GetVolumes

- For KAA_Authenticate_old / KAA_Authenticate, this can happen with
  the 'answer' argument in ka_Authenticate if KAA_AuthenticateV2 or
  KAA_Authenticate return RXGEN_OPCODE, but the server manages to
  populate oanswer.SeqLen with non-zero.

For all of these, make sure the memory is blanked before running the
relevant RPC. For ka_Authenticate, reset oanswer.SeqLen to 0 to avoid
sending any data, but still blank 'answer' and 'answer_old' just to be
safe.

FIXES 135043

Reviewed-on: https://gerrit.openafs.org/15925
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit c4e28c2afe743aa323be57ef3b0faec13027e678)

Change-Id: If44320c1efde98c53eed88099cd978ef89f4c0d8
Reviewed-on: https://gerrit.openafs.org/15947
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>