ipv6: fix per device IP snmp counters

In commit 4ce3c183 (snmp: 64bit ipstats_mib for all arches), I forgot
to change the /proc/net/dev_snmp6/xxx output for IP counters.

percpu array is 64bit per counter but the folding still used the 'long'
variant, and output garbage on 32bit arches.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/proc.c

@@ -237,8 +237,8 @@ static int snmp6_dev_seq_show(struct seq_file *seq, void *v)
 	struct inet6_dev *idev = (struct inet6_dev *)seq->private;
 
 	seq_printf(seq, "%-32s\t%u\n", "ifIndex", idev->dev->ifindex);
-	snmp6_seq_show_item(seq, (void __percpu **)idev->stats.ipv6, NULL,
-			    snmp6_ipstats_list);
+	snmp6_seq_show_item64(seq, (void __percpu **)idev->stats.ipv6,
+			    snmp6_ipstats_list, offsetof(struct ipstats_mib, syncp));
 	snmp6_seq_show_item(seq, NULL, idev->stats.icmpv6dev->mibs,
 			    snmp6_icmp6_list);
 	snmp6_seq_show_icmpv6msg(seq, idev->stats.icmpv6msgdev->mibs);