block, bfq: port of fix commits to (at least) 5.6

Hi, a bug reported for Fedora [1] goes away with the following fix commits, currently available from 5.7-rc1.

4d38a87fbb77 block, bfq: invoke flush_idle_tree after reparent_active_queues in pd_offline 576682fa52cb block, bfq: make reparent_leaf_entity actually work only on leaf entities c89977366500 block, bfq: turn put_queue into release_process_ref in __bfq_bic_change_cgroup

It would be useful for Fedora to have these fixes in (at least) 5.6. No change should be needed for these commits to apply cleanly.

Thanks, Paolo

[1] https://bugzilla.kernel.org/show_bug.cgi?id=205447#c84