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/sysupdate-transfer.c

@@ -989,7 +989,7 @@ static int helper_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;
 
         if (!pidref_equal(&ctx->pid, &sender_pid)) {
                 log_warning("Got notification datagram from unexpected peer, ignoring.");