snmp: fix OutOctets counter to include forwarded datagrams

RFC 4293 defines ipIfStatsOutOctets (similar definition for
ipSystemStatsOutOctets):

The total number of octets in IP datagrams delivered to the lower
layers for transmission. Octets from datagrams counted in
ipIfStatsOutTransmits MUST be counted here.

And ipIfStatsOutTransmits:

The total number of IP datagrams that this entity supplied to the
lower layers for transmission. This includes datagrams generated
locally and those forwarded by this entity.

Therefore, IPSTATS_MIB_OUTOCTETS must be incremented when incrementing
IPSTATS_MIB_OUTFORWDATAGRAMS.

IP_UPD_PO_STATS is not used since ipIfStatsOutRequests must not
include forwarded datagrams:

The total number of IP datagrams that local IP user-protocols
(including ICMP) supplied to IP in requests for transmission. Note
that this counter does not include any datagrams counted in
ipIfStatsOutForwDatagrams.

Signed-off-by: Vincent Bernat <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ip_forward.c

···
 	struct ip_options *opt	= &(IPCB(skb)->opt);
 
 	IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS);
+	IP_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTOCTETS, skb->len);
 
 	if (unlikely(opt->optlen))
 		ip_forward_options(skb);

net/ipv4/ipmr.c

···
 	struct ip_options *opt = &(IPCB(skb)->opt);
 
 	IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS);
+	IP_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTOCTETS, skb->len);
 
 	if (unlikely(opt->optlen))
 		ip_forward_options(skb);

net/ipv6/ip6_output.c

···
 	hdr->hop_limit--;
 
 	IP6_INC_STATS_BH(net, ip6_dst_idev(dst), IPSTATS_MIB_OUTFORWDATAGRAMS);
+	IP6_ADD_STATS_BH(net, ip6_dst_idev(dst), IPSTATS_MIB_OUTOCTETS, skb->len);
 	return NF_HOOK(NFPROTO_IPV6, NF_INET_FORWARD, skb, skb->dev, dst->dev,
 		       ip6_forward_finish);
 

net/ipv6/ip6mr.c

···
 {
 	IP6_INC_STATS_BH(dev_net(skb_dst(skb)->dev), ip6_dst_idev(skb_dst(skb)),
 			 IPSTATS_MIB_OUTFORWDATAGRAMS);
+	IP6_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), ip6_dst_idev(skb_dst(skb)),
+			 IPSTATS_MIB_OUTOCTETS, skb->len);
 	return dst_output(skb);
 }