pkt_sched: Use qdisc_lock() on already sampled root qdisc.

Based upon a bug report by Jeff Kirsher.

Don't use qdisc_root_lock() in these cases as the root
qdisc could have been changed, and we'd thus lock the
wrong object.

Tested by Emil S Tantilov who confirms that this seems
to fix the problem.

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