[PKT_SCHED] SFQ: timer is deferrable

The perturbation timer used for re-keying can be deferred, it doesn't
need to be deterministic.

Signed-off-by: Stephen Hemminger <stephen.hemminger@vyatta.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>