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/sysupdate/sysupdated.c

@@ -1657,7 +1657,7 @@ static int manager_on_notify(sd_event_source *s, int fd, uint32_t revents, void
         if (r == -EAGAIN)
                 return 0;
         if (r < 0)
-                return log_warning_errno(r, "Failed to receive notification message: %m");
+                return r;
 
         Job *j;
         HASHMAP_FOREACH(j, m->jobs) {