notify-recv: several followups

Follow-up for 7f6af95dab

- 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.

src/nspawn/nspawn.c

@@ -4626,7 +4626,7 @@ static int nspawn_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t r
         if (r == -EAGAIN)
                 return 0;
         if (r < 0)
-                return log_error_errno(r, "Failed to receive notification message: %m");
+                return r;
 
         if (sender_pid.pid != inner_child_pid) {
                 log_debug("Received notify message from process that is not the payload's PID 1. Ignoring.");