pkt_sched: Simplify dev_deactivate() polling loop.

The condition under which the previous qdisc has no more references
after we've attached &noop_qdisc is that both RUNNING and SCHED
are both seen clear while holding the root lock.

So just make specifically that check in the polling loop, instead
of this overly complex "check without then check with lock held"
sequence.

Signed-off-by: David S. Miller <davem@davemloft.net>