RCU fixes for v6.7

* Fix a lock inversion between scheduler and RCU introduced
  in v6.2-rc4. The scenario could trigger on any user of RCU_NOCB
  (mostly Android and also nohz_full).

* Fix PF_IDLE semantic changes introduced in v6.6-rc3 breaking some
  RCU-Tasks and RCU-Tasks-Trace expectations as to what exactly is
  an idle task. This resulted in potential spurious stalls and
  warnings.
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