ipv6 addrconf: fix preferred lifetime state-changing behavior while valid_lft is infinity

Fixed a problem with setting the lifetime of an IPv6
address. When setting preferred_lft to a value not zero or
infinity, while valid_lft is infinity(0xffffffff) preferred
lifetime is set to forever and does not update. Therefore
preferred lifetime never becomes deprecated. valid lifetime
and preferred lifetime should be set independently, even if
valid lifetime is infinity, preferred lifetime must expire
correctly (meaning it must eventually become deprecated)

Signed-off-by: Yasushi Asano <yasushi.asano@jp.fujitsu.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>