netfilter: ipt_CLUSTERIP: fix deadlock in netns exit routine

When network namespace is destroyed, cleanup_net() is called.
cleanup_net() holds pernet_ops_rwsem then calls each ->exit callback.
So that clusterip_tg_destroy() is called by cleanup_net().
And clusterip_tg_destroy() calls unregister_netdevice_notifier().

But both cleanup_net() and clusterip_tg_destroy() hold same
lock(pernet_ops_rwsem). hence deadlock occurrs.

After this patch, only 1 notifier is registered when module is inserted.
And all of configs are added to per-net list.

test commands:
   %ip netns add vm1
   %ip netns exec vm1 bash
   %ip link set lo up
   %iptables -A INPUT -p tcp -i lo -d 192.168.0.5 --dport 80 \
	-j CLUSTERIP --new --hashmode sourceip \
	--clustermac 01:00:5e:00:00:20 --total-nodes 2 --local-node 1
   %exit
   %ip netns del vm1

splat looks like:
[  341.809674] ============================================
[  341.809674] WARNING: possible recursive locking detected
[  341.809674] 4.19.0-rc5+ #16 Tainted: G        W
[  341.809674] --------------------------------------------
[  341.809674] kworker/u4:2/87 is trying to acquire lock:
[  341.809674] 000000005da2d519 (pernet_ops_rwsem){++++}, at: unregister_netdevice_notifier+0x8c/0x460
[  341.809674]
[  341.809674] but task is already holding lock:
[  341.809674] 000000005da2d519 (pernet_ops_rwsem){++++}, at: cleanup_net+0x119/0x900
[  341.809674]
[  341.809674] other info that might help us debug this:
[  341.809674]  Possible unsafe locking scenario:
[  341.809674]
[  341.809674]        CPU0
[  341.809674]        ----
[  341.809674]   lock(pernet_ops_rwsem);
[  341.809674]   lock(pernet_ops_rwsem);
[  341.809674]
[  341.809674]  *** DEADLOCK ***
[  341.809674]
[  341.809674]  May be due to missing lock nesting notation
[  341.809674]
[  341.809674] 3 locks held by kworker/u4:2/87:
[  341.809674]  #0: 00000000d9df6c92 ((wq_completion)"%s""netns"){+.+.}, at: process_one_work+0xafe/0x1de0
[  341.809674]  #1: 00000000c2cbcee2 (net_cleanup_work){+.+.}, at: process_one_work+0xb60/0x1de0
[  341.809674]  #2: 000000005da2d519 (pernet_ops_rwsem){++++}, at: cleanup_net+0x119/0x900
[  341.809674]
[  341.809674] stack backtrace:
[  341.809674] CPU: 1 PID: 87 Comm: kworker/u4:2 Tainted: G        W         4.19.0-rc5+ #16
[  341.809674] Workqueue: netns cleanup_net
[  341.809674] Call Trace:
[ ... ]
[  342.070196]  down_write+0x93/0x160
[  342.070196]  ? unregister_netdevice_notifier+0x8c/0x460
[  342.070196]  ? down_read+0x1e0/0x1e0
[  342.070196]  ? sched_clock_cpu+0x126/0x170
[  342.070196]  ? find_held_lock+0x39/0x1c0
[  342.070196]  unregister_netdevice_notifier+0x8c/0x460
[  342.070196]  ? register_netdevice_notifier+0x790/0x790
[  342.070196]  ? __local_bh_enable_ip+0xe9/0x1b0
[  342.070196]  ? __local_bh_enable_ip+0xe9/0x1b0
[  342.070196]  ? clusterip_tg_destroy+0x372/0x650 [ipt_CLUSTERIP]
[  342.070196]  ? trace_hardirqs_on+0x93/0x210
[  342.070196]  ? __bpf_trace_preemptirq_template+0x10/0x10
[  342.070196]  ? clusterip_tg_destroy+0x372/0x650 [ipt_CLUSTERIP]
[  342.123094]  clusterip_tg_destroy+0x3ad/0x650 [ipt_CLUSTERIP]
[  342.123094]  ? clusterip_net_init+0x3d0/0x3d0 [ipt_CLUSTERIP]
[  342.123094]  ? cleanup_match+0x17d/0x200 [ip_tables]
[  342.123094]  ? xt_unregister_table+0x215/0x300 [x_tables]
[  342.123094]  ? kfree+0xe2/0x2a0
[  342.123094]  cleanup_entry+0x1d5/0x2f0 [ip_tables]
[  342.123094]  ? cleanup_match+0x200/0x200 [ip_tables]
[  342.123094]  __ipt_unregister_table+0x9b/0x1a0 [ip_tables]
[  342.123094]  iptable_filter_net_exit+0x43/0x80 [iptable_filter]
[  342.123094]  ops_exit_list.isra.10+0x94/0x140
[  342.123094]  cleanup_net+0x45b/0x900
[ ... ]

Fixes: 202f59af ("netfilter: ipt_CLUSTERIP: do not hold dev")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>