ipv6: use addrconf_get_prefix_route() to remove peer addr

addrconf_get_prefix_route() ensures to get the right route in the right table.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>