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;

Change-Id: I2bdea2e808bb215720492b0ba6ac1a88da61b954
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>