block, bfq: consider also in_service_entity to state whether an entity is active
Groups of BFQ queues are represented by generic entities in BFQ. When a queue belonging to a parent entity is deactivated, the parent entity may need to be deactivated too, in case the deactivated queue was the only active queue for the parent entity. This deactivation may need to be propagated upwards if the entity belongs, in its turn, to a further higher-level entity, and so on. In particular, the upward propagation of deactivation stops at the first parent entity that remains active even if one of its child entities has been deactivated. To decide whether the last non-deactivation condition holds for a parent entity, BFQ checks whether the field next_in_service is still not NULL for the parent entity, after the deactivation of one of its child entity. If it is not NULL, then there are certainly other active entities in the parent entity, and deactivations can stop. Unfortunately, this check misses a corner case: if in_s...
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