tcp: stop stack timers in tcp_switch_back_to_default()

This funcion is an alternative code path that detaches an alternative
TCP stack, missed in d2ef52ef3dee38cccb7f54d33ecc2a4b944dad9d.

Reviewed by: rrs, tuexen
Differential Revision: https://reviews.freebsd.org/D42917
Reported-by: syzbot+186130be9f0ca5557d4e@syzkaller.appspotmail.com
Fixes: d2ef52ef3dee38cccb7f54d33ecc2a4b944dad9d

(cherry picked from commit ade05d63b727d5e8d0d833c1d974a9d50d4cb1bb)