pkt_sched: Rework {sch,tbf}_tree_lock().

Make sch_tree_lock() lock the qdisc's root.  All of the
users hold the RTNL semaphore and the root qdisc is not
changing.

Implement tbf_tree_{lock,unlock}() simply in terms of
sch_tree_{lock,unlock}().

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