netfilter: nf_tables: disable preemption in nft_update_chain_stats()

This patch fixes the following splat.

[118709.054937] BUG: using smp_processor_id() in preemptible [00000000] code: test/1571
[118709.054970] caller is nft_update_chain_stats.isra.4+0x53/0x97 [nf_tables]
[118709.054980] CPU: 2 PID: 1571 Comm: test Not tainted 4.17.0-rc6+ #335
[...]
[118709.054992] Call Trace:
[118709.055011]  dump_stack+0x5f/0x86
[118709.055026]  check_preemption_disabled+0xd4/0xe4
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_core.c

@@ -126,15 +126,15 @@ static noinline void nft_update_chain_stats(const struct nft_chain *chain,
 	if (!base_chain->stats)
 		return;
 
+	local_bh_disable();
 	stats = this_cpu_ptr(rcu_dereference(base_chain->stats));
 	if (stats) {
-		local_bh_disable();
 		u64_stats_update_begin(&stats->syncp);
 		stats->pkts++;
 		stats->bytes += pkt->skb->len;
 		u64_stats_update_end(&stats->syncp);
-		local_bh_enable();
 	}
+	local_bh_enable();
 }
 
 struct nft_jumpstack {