net_sched: do not reprogram a timer about to expire

qdisc_watchdog_schedule_range_ns() can use the newly added slack
and avoid rearming the hrtimer a bit earlier than the current
value. This patch has no effect if delta_ns parameter
is zero.

Note that this means the max slack is potentially doubled.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>