netfilter: nf_tables: increase nft_counters_enabled in nft_chain_stats_replace()

When a chain is updated, a counter can be attached. if so,
the nft_counters_enabled should be increased.

test commands:

   %nft add table ip filter
   %nft add chain ip filter input { type filter hook input priority 4\; }
   %iptables-compat -Z input
   %nft delete chain ip filter input

we can see below messages.

[  286.443720] jump label: negative count!
[  286.448278] WARNING: CPU: 0 PID: 1459 at kernel/jump_label.c:197 __static_key_slow_dec_cpuslocked+0x6f/0xf0
[  286.449144] Modules linked in: nf_tables nfnetlink ip_tables x_tables
[  286.449144] CPU: 0 PID: 1459 Comm: nft Tainted: G        W         4.17.0-rc2+ #12
[  286.449144] RIP: 0010:__static_key_slow_dec_cpuslocked+0x6f/0xf0
[  286.449144] RSP: 0018:ffff88010e5176f0 EFLAGS: 00010286
[  286.449144] RAX: 000000000000001b RBX: ffffffffc0179500 RCX: ffffffffb8a82522
[  286.449144] RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff88011b7e5eac
[  286.449144] RBP: 0000000000000000 R08: ffffed00236fce5c R09: ffffed00236fce5b
[  286.449144] R10: ffffffffc0179503 R11: ffffed00236fce5c R12: 0000000000000000
[  286.449144] R13: ffff88011a28e448 R14: ffff88011a28e470 R15: dffffc0000000000
[  286.449144] FS:  00007f0384328700(0000) GS:ffff88011b600000(0000) knlGS:0000000000000000
[  286.449144] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  286.449144] CR2: 00007f038394bf10 CR3: 0000000104a86000 CR4: 00000000001006f0
[  286.449144] Call Trace:
[  286.449144]  static_key_slow_dec+0x6a/0x70
[  286.449144]  nf_tables_chain_destroy+0x19d/0x210 [nf_tables]
[  286.449144]  nf_tables_commit+0x1891/0x1c50 [nf_tables]
[  286.449144]  nfnetlink_rcv+0x1148/0x13d0 [nfnetlink]
[ ... ]
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1298,8 +1298,10 @@ static void nft_chain_stats_replace(struct nft_base_chain *chain,
 		rcu_assign_pointer(chain->stats, newstats);
 		synchronize_rcu();
 		free_percpu(oldstats);
-	} else
+	} else {
 		rcu_assign_pointer(chain->stats, newstats);
+		static_branch_inc(&nft_counters_enabled);
+	}
 }
 
 static void nf_tables_chain_destroy(struct nft_ctx *ctx)