[IPV6]: Track device renames in snmp6.

When network device's are renamed, the IPV6 snmp6 code
gets confused. It doesn't track name changes so it will OOPS
when network device's are removed.

The fix is trivial, just unregister/re-register in notify handler.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>