ipv6: Compare addresses only bits up to the prefix length (RFC6724).

Compare bits up to the source address's prefix length only to
allows DNS load balancing to continue to be used as a tie breaker.

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