ipv6: snmp: do not track per idev ICMP6_MIB_RATELIMITHOST

Blamed commit added a critical false sharing on a single
atomic_long_t under DOS, like receiving UDP packets
to closed ports.

Per netns ICMP6_MIB_RATELIMITHOST tracking uses per-cpu
storage and is enough, we do not need per-device and slow tracking.

Fixes: d0941130c9351 ("icmp: Add counters for rate limits")
Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Jamie Bainbridge <[e-mail]>
Cc: Abhishek Rawal <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv6/icmp.c

···
 	}
 	rcu_read_unlock();
 	if (!res)
-		__ICMP6_INC_STATS(net, ip6_dst_idev(dst),
-				  ICMP6_MIB_RATELIMITHOST);
+		__ICMP6_INC_STATS(net, NULL, ICMP6_MIB_RATELIMITHOST);
 	else
 		icmp_global_consume(net);
 	dst_release(dst);

net/ipv6/proc.c

···
 	SNMP_MIB_ITEM("Icmp6OutMsgs", ICMP6_MIB_OUTMSGS),
 	SNMP_MIB_ITEM("Icmp6OutErrors", ICMP6_MIB_OUTERRORS),
 	SNMP_MIB_ITEM("Icmp6InCsumErrors", ICMP6_MIB_CSUMERRORS),
+/* ICMP6_MIB_RATELIMITHOST needs to be last, see snmp6_dev_seq_show(). */
 	SNMP_MIB_ITEM("Icmp6OutRateLimitHost", ICMP6_MIB_RATELIMITHOST),
 };
 
···
 			      snmp6_ipstats_list,
 			      ARRAY_SIZE(snmp6_ipstats_list),
 			      offsetof(struct ipstats_mib, syncp));
+
+	/* Per idev icmp stats do not have ICMP6_MIB_RATELIMITHOST */
 	snmp6_seq_show_item(seq, NULL, idev->stats.icmpv6dev->mibs,
-			    snmp6_icmp6_list, ARRAY_SIZE(snmp6_icmp6_list));
+			    snmp6_icmp6_list, ARRAY_SIZE(snmp6_icmp6_list) - 1);
+
 	snmp6_seq_show_icmpv6msg(seq, idev->stats.icmpv6msgdev->mibs);
 	return 0;
 }