BACKPORT: sched: Fix yet more sched_fork() races

Where commit 4ef0c5c6 ("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 13765de8 ("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.

Change-Id: If8d8fc38f2ab1334c45e530aaaedfc7b2825a7e5
Fixes: 4ef0c5c6 ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Tested-by: Zhang Qiao <zhangqiao22@huawei.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lkml.kernel.org/r/YgoeCbwj5mbCR0qA@hirez.programming.kicks-ass.net


BUG: 221793230
(cherry picked from commit b1e82065)
[quic_ashayj: Resolved minor merge conflicts]
Signed-off-by: Ashay Jaiswal <quic_ashayj@quicinc.com>