pkt_sched: Add queue stopped test back to qdisc_run().

Based upon a bug report by Andrew Gallatin on netdev
with subject "CPU utilization increased in 2.6.27rc"

In commit 37437bb2
("pkt_sched: Schedule qdiscs instead of netdev_queue.")
the test of the queue being stopped was erroneously
removed from qdisc_run().

When the TX queue of the device fills up, this omission
causes lots of extraneous useless work to be queued up
to softirq context, where we'll just return immediately
because the device is still stuffed up.

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