blkio: Keep queue on service tree until we expire it

o Currently cfqq deletes a queue from service tree if it is empty (even if
  we might idle on the queue). This patch keeps the queue on service tree
  hence associated group remains on the service tree until we decide that
  we are not going to idle on the queue and expire it.

o This just helps in time accounting for queue/group and in implementation
  of rest of the patches.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>