net_sched: introduce tcf_exts_get_net() and tcf_exts_put_net()

Instead of holding netns refcnt in tc actions, we can minimize
the holding time by saving it in struct tcf_exts instead. This
means we can just hold netns refcnt right before call_rcu() and
release it after tcf_exts_destroy() is done.

However, because on netns cleanup path we call tcf_proto_destroy()
too, obviously we can not hold netns for a zero refcnt, in this
case we have to do cleanup synchronously. It is fine for RCU too,
the caller cleanup_net() already waits for a grace period.

For other cases, refcnt is non-zero and we can safely grab it as
normal and release it after we are done.

This patch provides two new API for each filter to use:
tcf_exts_get_net() and tcf_exts_put_net(). And all filters now can
use the following pattern:

void __destroy_filter() {
  tcf_exts_destroy();
  tcf_exts_put_net();  // <== release netns refcnt
  kfree();
}
void some_work() {
  rtnl_lock();
  __destroy_filter();
  rtnl_unlock();
}
void some_rcu_callback() {
  tcf_queue_work(some_work);
}

if (tcf_exts_get_net())  // <== hold netns refcnt
  call_rcu(some_rcu_callback);
else
  __destroy_filter();

Cc: Lucas Bates <lucasb@mojatatu.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>