SUNRPC: Convert unwrap data paths to use xdr_stream for replies

We're now moving svcxdr_init_encode() to /before/ the flavor's
->accept method has set rq_auth_slack. Add a helper that can
set rq_auth_slack /after/ svcxdr_init_encode() has been called.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>