OPENAFS-SA-2019-002: Zero all server RPC args

OPENAFS-SA-2019-002: Zero all server RPC args

Currently, our server-side RPC argument-handling code generated from
rxgen initializes complex arguments like so (for example, in
_RXAFS_BulkStatus):

    AFSCBFids FidsArray;
    AFSBulkStats StatArray;
    AFSCBs CBArray;
    AFSVolSync Sync;

    FidsArray.AFSCBFids_val = 0;
    FidsArray.AFSCBFids_len = 0;
    CBArray.AFSCBs_val = 0;
    CBArray.AFSCBs_len = 0;
    StatArray.AFSBulkStats_val = 0;
    StatArray.AFSBulkStats_len = 0;

This is done for any input or output arguments, but only for types we
need to free afterwards (arrays, usually). We do not do this for
simple types, like single flat structs. In the above example, we do
this for the arrays FidsArray, StatArray, and CBArray, but 'Sync' is
not initialized to anything.

If some server RPC handlers never set a value for an output argument,
this means we'll send uninitialized stack memory to our peer.
Currently this can happen in, for example,
MRXSTATS_RetrieveProcessRPCStats if 'rxi_monitor_processStats' is
unset (specifically, the 'clock_sec' and 'clock_usec' arguments are
never set when rx_enableProcessRPCStats() has not been called).

To make sure we cannot send uninitialized data to our peer, change
rxgen to instead 'memset(&arg, 0, sizeof(arg));' for every single
parameter. Using memset in this way just makes this a little simpler
inside rxgen, since all we need to do this is the name of the
argument.

With this commit, the rxgen-generated code for the above example now
looks like this:

    AFSCBFids FidsArray;
    AFSBulkStats StatArray;
    AFSCBs CBArray;
    AFSVolSync Sync;

    memset(&FidsArray, 0, sizeof(FidsArray));
    memset(&CBArray, 0, sizeof(CBArray));
    memset(&StatArray, 0, sizeof(StatsArray));
    memset(&Sync, 0, sizeof(Sync));

Reviewed-on: https://gerrit.openafs.org/13914
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 93aee3cf40622993b95bd1af77080a31670c24bb)

Change-Id: I6e19aaea57e545455b65851d1bedade584e482f0
Reviewed-on: https://gerrit.openafs.org/13917
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>