notify-recv: several followups

notify-recv: several followups

Follow-up for 7f6af95dab037e7d15591a924dbf256460bbf069

- Allocate internal buf on the stack, memdup() only at the end.
  This ensures we're able to handle OOM gracefully, i.e.
  return -EAGAIN on OOM while still emptying socket buffer.
- Do not treat empty notify message as error.
- Raise log level since all callers log loudly anyway.