ipv6 route: Fix lifetime in netlink.

We could not see appropriate lifetime if the route had been scheduled
to expired at 0 (in jiffies).  We should check rt6i_flags instead of
rt6i_expires to determine whether lifetime is valid or not.

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