Please apply f6783319737f ('sched/fair: Fix insertion in rq->leaf_cfs_rq_list') to 4.19.y

The f6783319737f appears to fix a reproducible crash that we have been experiencing while stress-testing creation and deletion of containers.

The fix seems to apply to 4.19.y more easily if you first apply also 5d299eabea5a ('sched/fair: Add tmp_alone_branch assertion').