net_sched: add a temporary refcnt for struct tcindex_data

Although we intentionally use an ordered workqueue for all tc
filter works, the ordering is not guaranteed by RCU work,
given that tcf_queue_work() is esstenially a call_rcu().

This problem is demostrated by Thomas:

  CPU 0:
    tcf_queue_work()
      tcf_queue_work(&r->rwork, tcindex_destroy_rexts_work);

  -> Migration to CPU 1

  CPU 1:
     tcf_queue_work(&p->rwork, tcindex_destroy_work);

so the 2nd work could be queued before the 1st one, which leads
to a free-after-free.

Enforcing this order in RCU work is hard as it requires to change
RCU code too. Fortunately we can workaround this problem in tcindex
filter by taking a temporary refcnt, we only refcnt it right before
we begin to destroy it. This simplifies the code a lot as a full
refcnt requires much more changes in tcindex_set_parms().

Reported-by:  <syzbot+46f513c3033d592409d2@syzkaller.appspotmail.com>
Fixes: 3d210534

 ("net_sched: fix a race condition in tcindex_destroy()")
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>