[IPv6] SNMP: Increment OutNoRoutes when connecting to unreachable network

IPv6 stack doesn't increment OutNoRoutes counter when IP datagrams
is being discarded because no route could be found to transmit them
to their destination. IPv6 stack should increment the counter.
Incidentally, IPv4 stack increments that counter in such situation.

Signed-off-by: Mitsuru Chinen <mitch@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>