[IPV6]: ADDRCONF: Use our standard algorithm for randomized ifid.

RFC 3041 describes an algorithm to generate random interface
identifier.  In RFC 3041bis, it is allowed to use different
algorithm than one described in RFC 3041.

So, let's use our standard pseudo random algorithm to simplify
our implementation.

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