[IPV6] SNMP: Fix {In,Out}NoRoutes statistics.

A packet which is being discarded because of no routes in the
forwarding path should not be counted as OutNoRoutes but as
InNoRoutes.
Additionally, on this occasion, a packet whose destinaion is
not valid should be counted as InAddrErrors separately.

Based on patch from Mitsuru Chinen <mitch@linux.vnet.ibm.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>