net: ipv4, ipv6: fix IPSTATS_MIB_OUTOCTETS increment duplicated

commit edf391ff ("snmp: add missing counters for RFC 4293") had
already added OutOctets for RFC 4293. In commit 2d8dbb04 ("snmp: fix
OutOctets counter to include forwarded datagrams"), OutOctets was
counted again, but not removed from ip_output().

According to RFC 4293 "3.2.3. IP Statistics Tables",
ipipIfStatsOutTransmits is not equal to ipIfStatsOutForwDatagrams. So
"IPSTATS_MIB_OUTOCTETS must be incremented when incrementing" is not
accurate. And IPSTATS_MIB_OUTOCTETS should be counted after fragment.

This patch reverts commit 2d8dbb04 ("snmp: fix OutOctets counter to
include forwarded datagrams") and move IPSTATS_MIB_OUTOCTETS to
ip_finish_output2 for ipv4.

Reviewed-by: Filip Pudak <filip.pudak@windriver.com>
Signed-off-by: Heng Guo <heng.guo@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>