net/sched: cls_api: Fix lockup on flushing explicitly created chain

Mingshuai Ren reports:

When a new chain is added by using tc, one soft lockup alarm will be
 generated after delete the prio 0 filter of the chain. To reproduce
 the problem, perform the following steps:
(1) tc qdisc add dev eth0 root handle 1: htb default 1
(2) tc chain add dev eth0
(3) tc filter del dev eth0 chain 0 parent 1: prio 0
(4) tc filter add dev eth0 chain 0 parent 1:

Fix the issue by accounting for additional reference to chains that are
explicitly created by RTM_NEWCHAIN message as opposed to implicitly by
RTM_NEWTFILTER message.

Fixes: 726d0612

 ("net: sched: prevent insertion of new classifiers during chain flush")
Reported-by: Mingshuai Ren <renmingshuai@huawei.com>
Closes: https://lore.kernel.org/lkml/87legswvi3.fsf@nvidia.com/T/
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Link: https://lore.kernel.org/r/20230612093426.2867183-1-vladbu@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>