[IPV6] ADDRCONF: Support RFC3484 configurable address selection policy table.

Policy table is implemented as an RCU linear list since we do not expect
large list nor frequent updates.

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