net_sched/sfq: update hierarchical backlog when drop packet

When sfq_enqueue() drops head packet or packet from another queue it
have to update backlog at upper qdiscs too.

Fixes: 2ccccf5f

 ("net_sched: update hierarchical backlog too")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>