netfilter: nf_tables: check the result of dereferencing base_chain->stats

Check the result of dereferencing base_chain->stats, instead of result
of this_cpu_ptr with NULL.

base_chain->stats maybe be changed to NULL when a chain is updated and a
new NULL counter can be attached.

And we do not need to check returning of this_cpu_ptr since
base_chain->stats is from percpu allocator if it is non-NULL,
this_cpu_ptr returns a valid value.

And fix two sparse error by replacing rcu_access_pointer and
rcu_dereference with READ_ONCE under rcu_read_lock.

Thanks for Eric's help to finish this patch.

Fixes: 00924094 ("netfilter: nf_tables: don't assume chain stats are set when jumplabel is set")
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_core.c

@@ -98,21 +98,23 @@ static noinline void nft_update_chain_stats(const struct nft_chain *chain,
 					    const struct nft_pktinfo *pkt)
 {
 	struct nft_base_chain *base_chain;
+	struct nft_stats __percpu *pstats;
 	struct nft_stats *stats;
 
 	base_chain = nft_base_chain(chain);
-	if (!rcu_access_pointer(base_chain->stats))
-		return;
 
+	rcu_read_lock();
+	pstats = READ_ONCE(base_chain->stats);
+	if (pstats) {
 		local_bh_disable();
-	stats = this_cpu_ptr(rcu_dereference(base_chain->stats));
-	if (stats) {
+		stats = this_cpu_ptr(pstats);
 		u64_stats_update_begin(&stats->syncp);
 		stats->pkts++;
 		stats->bytes += pkt->skb->len;
 		u64_stats_update_end(&stats->syncp);
-	}
 		local_bh_enable();
+	}
+	rcu_read_unlock();
 }
 
 struct nft_jumpstack {