ipv6: fix the bug of address check

The duplicate address check code got broken in the conversion
to hlist (2.6.35).  The earlier patch did not fix the case where
two addresses match same hash value. Use two exit paths,
rather than depending on state of loop variables (from macro).

Based on earlier fix by Shan Wei.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Reviewed-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>