Play nice with other qsbr users

This hack has been working fine for years, but that's because it's the only vmod that happens to use urcu-qsbr. If more than one vmod were to use it, we'd see individual threads registering and unregistering multiple times redundantly at the global urcu level, and unfortunately urcu-qsbr's register/unregister calls are not idempotent and this would cause all kinds of subtle issues.