ndisc: Add missing strategies for per-device retrans timer/reachable time settings.

Noticed from Al Viro <viro@ftp.linux.org.uk> via David Miller
<davem@davemloft.net>.

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