sch_api: introduce qdisc_watchdog_schedule_ns()

tbf will need to schedule watchdog in ns. No need to convert it twice.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>