ipv6: use round_jiffies

This timer normally happens once a minute, there is no need to cause an
early wakeup for it, so align it to next second boundary to safe power.
It can't be deferred because then it could take too long on cleanup or DoS.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>