pkt_sched: Convert qdisc_watchdog to tasklet_hrtimer

None of this stuff should execute in hw IRQ context, therefore
use a tasklet_hrtimer so that it runs in softirq context.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Gleixner <tglx@linutronix.de>