OPENAFS-SA-2019-001: Skip server OUT args on error

OPENAFS-SA-2019-001: Skip server OUT args on error

Currently, part of our server-side RPC argument-handling code that's
generated from rxgen looks like this (for example):

    z_result = SRXAFS_BulkStatus(z_call, &FidsArray, &StatArray, &CBArray, &Sync);
    z_xdrs->x_op = XDR_ENCODE;
    if ((!xdr_AFSBulkStats(z_xdrs, &StatArray))
         || (!xdr_AFSCBs(z_xdrs, &CBArray))
         || (!xdr_AFSVolSync(z_xdrs, &Sync)))
            z_result = RXGEN_SS_MARSHAL;
fail:
    [...]
    return z_result;

When the server routine for implementing the RPC results a non-zero
value into z_result, the call will be aborted. However, before we
abort the call, we still call the xdr_* routines with XDR_ENCODE for
all of our output arguments. If the call has not already been aborted
for other reasons, we'll serialize the output argument data into the
Rx call. If we push more data than can fit in a single Rx packet for
the call, then we'll also send that data to the client. Many server
routines for implementing RPCs do not initialize the memory inside
their output arguments during certain errors, and so the memory may be
leaked to the peer.

To avoid this, just jump to the 'fail' label when a nonzero 'z_result'
is returned. This means we skip sending the output argument data to
the peer, but we still free any argument data that needs freeing, and
record the stats for the call (if needed). This makes the above
example now look like this:

    z_result = SRXAFS_BulkStatus(z_call, &FidsArray, &StatArray, &CBArray, &Sync);
    if (z_result)
        goto fail;
    z_xdrs->x_op = XDR_ENCODE;
    if ((!xdr_AFSBulkStats(z_xdrs, &StatArray))
         || (!xdr_AFSCBs(z_xdrs, &CBArray))
         || (!xdr_AFSVolSync(z_xdrs, &Sync)))
            z_result = RXGEN_SS_MARSHAL;
fail:
    [...]
    return z_result;

Reviewed-on: https://gerrit.openafs.org/13913
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit ea276e83e37e5bd27285a3d639f2158639172786)

Reviewed-on: https://gerrit.openafs.org/13916
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 5a3d1b62810fc8cc7b37a737b4f5f1912bc614f9)

Change-Id: Ia54b06399b1f8a05a560ec7560580783d3e64b9d
Reviewed-on: https://gerrit.openafs.org/13921
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>