xt_rateest_net_exit() was added to check whether rules are flushed
successfully. but ->net_exit() callback is called earlier than
->destroy() callback.
So that ->net_exit() callback can't check that.

test commands:
   %ip netns add vm1
   %ip netns exec vm1 iptables -t mangle -I PREROUTING -p udp \
	   --dport 1111 -j RATEEST --rateest-name ap \
	   --rateest-interval 250ms --rateest-ewma 0.5s
   %ip netns del vm1

splat looks like:
[  668.813518] WARNING: CPU: 0 PID: 87 at net/netfilter/xt_RATEEST.c:210 xt_rateest_net_exit+0x210/0x340 [xt_RATEEST]
[  668.813518] Modules linked in: xt_RATEEST xt_tcpudp iptable_mangle bpfilter ip_tables x_tables
[  668.813518] CPU: 0 PID: 87 Comm: kworker/u4:2 Not tainted 4.19.0-rc7+ #21
[  668.813518] Workqueue: netns cleanup_net
[  668.813518] RIP: 0010:xt_rateest_net_exit+0x210/0x340 [xt_RATEEST]
[  668.813518] Code: 00 48 8b 85 30 ff ff ff 4c 8b 23 80 38 00 0f 85 24 01 00 00 48 8b 85 30 ff ff ff 4d 85 e4 4c 89 a5 58 ff ff ff c6 00 f8 74 b2 <0f> 0b 48 83 c3 08 4c 39 f3 75 b0 48 b8 00 00 00 00 00 fc ff df 49
[  668.813518] RSP: 0018:ffff8801156c73f8 EFLAGS: 00010282
[  668.813518] RAX: ffffed0022ad8e85 RBX: ffff880118928e98 RCX: 5db8012a00000000
[  668.813518] RDX: ffff8801156c7428 RSI: 00000000cb1d185f RDI: ffff880115663b74
[  668.813518] RBP: ffff8801156c74d0 R08: ffff8801156633c0 R09: 1ffff100236440be
[  668.813518] R10: 0000000000000001 R11: ffffed002367d852 R12: ffff880115142b08
[  668.813518] R13: 1ffff10022ad8e81 R14: ffff880118928ea8 R15: dffffc0000000000
[  668.813518] FS:  0000000000000000(0000) GS:ffff88011b200000(0000) knlGS:0000000000000000
[  668.813518] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  668.813518] CR2: 0000563aa69f4f28 CR3: 0000000105a16000 CR4: 00000000001006f0
[  668.813518] Call Trace:
[  668.813518]  ? unregister_netdevice_many+0xe0/0xe0
[  668.813518]  ? xt_rateest_net_init+0x2c0/0x2c0 [xt_RATEEST]
[  668.813518]  ? default_device_exit+0x1ca/0x270
[  668.813518]  ? remove_proc_entry+0x1cd/0x390
[  668.813518]  ? dev_change_net_namespace+0xd00/0xd00
[  668.813518]  ? __init_waitqueue_head+0x130/0x130
[  668.813518]  ops_exit_list.isra.10+0x94/0x140
[  668.813518]  cleanup_net+0x45b/0x900
[  668.813518]  ? net_drop_ns+0x110/0x110
[  668.813518]  ? swapgs_restore_regs_and_return_to_usermode+0x3c/0x80
[  668.813518]  ? save_trace+0x300/0x300
[  668.813518]  ? lock_acquire+0x196/0x470
[  668.813518]  ? lock_acquire+0x196/0x470
[  668.813518]  ? process_one_work+0xb60/0x1de0
[  668.813518]  ? _raw_spin_unlock_irq+0x29/0x40
[  668.813518]  ? _raw_spin_unlock_irq+0x29/0x40
[  668.813518]  ? __lock_acquire+0x4500/0x4500
[  668.813518]  ? __lock_is_held+0xb4/0x140
[  668.813518]  process_one_work+0xc13/0x1de0
[  668.813518]  ? pwq_dec_nr_in_flight+0x3c0/0x3c0
[  668.813518]  ? set_load_weight+0x270/0x270
[ ... ]

Fixes: 3427b2ab

 ("netfilter: make xt_rateest hash table per net")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>