qdisc_tree_decrease_qlen() is called when some packets are dropped
on a qdisc, and we want to notify parents of qlen changes.

We also can increment parents qdisc qstats drop counters.

This permits more accurate drop counters up to root qdisc.

For example a graft operation typically resets a qdisc
(drops all packets) and call qdisc_tree_decrease_qlen()

Note that callers are responsible for their drop counters.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>