netfilter: nf_conncount: fix list_del corruption in conn_free

nf_conncount_tuple is an element of nft_connlimit and that is deleted by
conn_free(). Elements can be deleted by both GC routine and data path
functions (nf_conncount_lookup, nf_conncount_add) and they call
conn_free() to free elements. But conn_free() only protects lists, not
each element. So that list_del corruption could occurred.

The conn_free() doesn't check whether element is already deleted. In
order to protect elements, dead flag is added. If an element is deleted,
dead flag is set. The only conn_free() can delete elements so that both
list lock and dead flag are enough to protect it.

test commands:
   %nft add table ip filter
   %nft add chain ip filter input { type filter hook input priority 0\; }
   %nft add rule filter input meter test { ip id ct count over 2 } counter

splat looks like:
[ 1779.495778] list_del corruption, ffff8800b6e12008->prev is LIST_POISON2 (dead000000000200)
[ 1779.505453] ------------[ cut here ]------------
[ 1779.506260] kernel BUG at lib/list_debug.c:50!
[ 1779.515831] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[ 1779.516772] CPU: 0 PID: 33 Comm: kworker/0:2 Not tainted 4.19.0-rc6+ #22
[ 1779.516772] Workqueue: events_power_efficient nft_rhash_gc [nf_tables_set]
[ 1779.516772] RIP: 0010:__list_del_entry_valid+0xd8/0x150
[ 1779.516772] Code: 39 48 83 c4 08 b8 01 00 00 00 5b 5d c3 48 89 ea 48 c7 c7 00 c3 5b 98 e8 0f dc 40 ff 0f 0b 48 c7 c7 60 c3 5b 98 e8 01 dc 40 ff <0f> 0b 48 c7 c7 c0 c3 5b 98 e8 f3 db 40 ff 0f 0b 48 c7 c7 20 c4 5b
[ 1779.516772] RSP: 0018:ffff880119127420 EFLAGS: 00010286
[ 1779.516772] RAX: 000000000000004e RBX: dead000000000200 RCX: 0000000000000000
[ 1779.516772] RDX: 000000000000004e RSI: 0000000000000008 RDI: ffffed0023224e7a
[ 1779.516772] RBP: ffff88011934bc10 R08: ffffed002367cea9 R09: ffffed002367cea9
[ 1779.516772] R10: 0000000000000001 R11: ffffed002367cea8 R12: ffff8800b6e12008
[ 1779.516772] R13: ffff8800b6e12010 R14: ffff88011934bc20 R15: ffff8800b6e12008
[ 1779.516772] FS:  0000000000000000(0000) GS:ffff88011b200000(0000) knlGS:0000000000000000
[ 1779.516772] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1779.516772] CR2: 00007fc876534010 CR3: 000000010da16000 CR4: 00000000001006f0
[ 1779.516772] Call Trace:
[ 1779.516772]  conn_free+0x9f/0x2b0 [nf_conncount]
[ 1779.516772]  ? nf_ct_tmpl_alloc+0x2a0/0x2a0 [nf_conntrack]
[ 1779.516772]  ? nf_conncount_add+0x520/0x520 [nf_conncount]
[ 1779.516772]  ? do_raw_spin_trylock+0x1a0/0x1a0
[ 1779.516772]  ? do_raw_spin_trylock+0x10/0x1a0
[ 1779.516772]  find_or_evict+0xe5/0x150 [nf_conncount]
[ 1779.516772]  nf_conncount_gc_list+0x162/0x360 [nf_conncount]
[ 1779.516772]  ? nf_conncount_lookup+0xee0/0xee0 [nf_conncount]
[ 1779.516772]  ? _raw_spin_unlock_irqrestore+0x45/0x50
[ 1779.516772]  ? trace_hardirqs_off+0x6b/0x220
[ 1779.516772]  ? trace_hardirqs_on_caller+0x220/0x220
[ 1779.516772]  nft_rhash_gc+0x16b/0x540 [nf_tables_set]
[ ... ]

Fixes: 5c789e13 ("netfilter: nf_conncount: Add list lock and gc worker, and RCU for init tree search")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>