[IPV6]: RFC 2011 compatibility broken

The snmp6 entry name was changed, and it broke compatibility
to RFC 2011.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>