tun: don't leak the QueueSet shutdown eventfd when Add fails

Both newTunGeneric error branches closed only the tun fd; the
Add-failure branch left the freshly created QueueSet (and its
shutdown eventfd) orphaned.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ugV2edVqoz3tBvq9J6yWp

overlay/tun_linux.go

@@ -242,7 +242,10 @@ func newTunGeneric(c *config.C, l *slog.Logger, fd int, vnetHdr bool, offloadFla
 	}
 	err = qs.Add(fd)
 	if err != nil {
+		// Add only appends on success, so closing the set here can't
+		// double-close fd; it releases the set's shutdown eventfd.
 		_ = unix.Close(fd)
+		_ = qs.Close()
 		return nil, err
 	}