ipv6: unneccessary to get address prefix in addrconf_get_prefix_route

Since addrconf_get_prefix_route inputs the address prefix to fib6_locate,
which does not uses the data which is out of the prefix_len length,
so do not need to use ipv6_addr_prefix to get address prefix.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>