net: sched: fix crash when deleting secondary chains

commit d7aa04a5e82b4f254d306926c81eae8df69e5200 upstream.

If you flush (delete) a filter chain other than chain 0 (such as when
deleting the device), the kernel may run into a use-after-free. The
chain refcount must not be decremented unless we are sure we are done
with the chain.

To reproduce the bug, run:
    ip link add dtest type dummy
    tc qdisc add dev dtest ingress
    tc filter add dev dtest chain 1  parent ffff: flower
    ip link del dtest

Introduced in: commit f93e1cdc ("net/sched: fix filter flushing"),
but unless you have KAsan or luck, you won't notice it until
commit 0dadc117 ("cls_flower: use tcf_exts_get_net() before call_rcu()")

Fixes: f93e1cdc ("net/sched: fix filter flushing")
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Roman Kapl <code@rkapl.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>