netfilter: ipt_CLUSTERIP: remove wrong WARN_ON_ONCE in netns exit routine

When network namespace is destroyed, both clusterip_tg_destroy() and
clusterip_net_exit() are called. and clusterip_net_exit() is called
before clusterip_tg_destroy().
Hence cleanup check code in clusterip_net_exit() doesn't make sense.

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:
[  341.184508] WARNING: CPU: 1 PID: 87 at net/ipv4/netfilter/ipt_CLUSTERIP.c:840 clusterip_net_exit+0x319/0x380 [ipt_CLUSTERIP]
[  341.184850] Modules linked in: ipt_CLUSTERIP nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 xt_tcpudp iptable_filter bpfilter ip_tables x_tables
[  341.184850] CPU: 1 PID: 87 Comm: kworker/u4:2 Not tainted 4.19.0-rc5+ #16
[  341.227509] Workqueue: netns cleanup_net
[  341.227509] RIP: 0010:clusterip_net_exit+0x319/0x380 [ipt_CLUSTERIP]
[  341.227509] Code: 0f 85 7f fe ff ff 48 c7 c2 80 64 2c c0 be a8 02 00 00 48 c7 c7 a0 63 2c c0 c6 05 18 6e 00 00 01 e8 bc 38 ff f5 e9 5b fe ff ff <0f> 0b e9 33 ff ff ff e8 4b 90 50 f6 e9 2d fe ff ff 48 89 df e8 de
[  341.227509] RSP: 0018:ffff88011086f408 EFLAGS: 00010202
[  341.227509] RAX: dffffc0000000000 RBX: 1ffff1002210de85 RCX: 0000000000000000
[  341.227509] RDX: 1ffff1002210de85 RSI: ffff880110813be8 RDI: ffffed002210de58
[  341.227509] RBP: ffff88011086f4d0 R08: 0000000000000000 R09: 0000000000000000
[  341.227509] R10: 0000000000000000 R11: 0000000000000000 R12: 1ffff1002210de81
[  341.227509] R13: ffff880110625a48 R14: ffff880114cec8c8 R15: 0000000000000014
[  341.227509] FS:  0000000000000000(0000) GS:ffff880116600000(0000) knlGS:0000000000000000
[  341.227509] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  341.227509] CR2: 00007f11fd38e000 CR3: 000000013ca16000 CR4: 00000000001006e0
[  341.227509] Call Trace:
[  341.227509]  ? __clusterip_config_find+0x460/0x460 [ipt_CLUSTERIP]
[  341.227509]  ? default_device_exit+0x1ca/0x270
[  341.227509]  ? remove_proc_entry+0x1cd/0x390
[  341.227509]  ? dev_change_net_namespace+0xd00/0xd00
[  341.227509]  ? __init_waitqueue_head+0x130/0x130
[  341.227509]  ops_exit_list.isra.10+0x94/0x140
[  341.227509]  cleanup_net+0x45b/0x900
[ ... ]

Fixes: 613d0776 ("netfilter: exit_net cleanup check added")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>