sched: Fix yet more sched_fork() races

In the Linux kernel, the following vulnerability has been resolved:

sched: Fix yet more sched_fork() races

Where commit 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an

invalid sched_task_group") fixed a fork race vs cgroup, it opened up a

race vs syscalls by not placing the task on the runqueue before it

gets exposed through the pidhash.

Commit 13765de8148f ("sched/fair: Fix fault in reweight_entity") is

trying to fix a single instance of this, instead fix the whole class

of issues, effectively reverting this commit.