tick/rcu: Stop allowing RCU_SOFTIRQ in idle

[Bodong Wang]

* Explain the bug
    RCU_SOFTIRQ used to be special in that it could be raised on purpose
    within the idle path to prevent from stopping the tick. Some code still
    prevents from unnecessary warnings related to this specific behaviour
    while entering in dynticks-idle mode.

    However the nohz layout has changed quite a bit in ten years, and the
    removal of CONFIG_RCU_FAST_NO_HZ has been the final straw to this
    safe-conduct. Now the RCU_SOFTIRQ vector is expected to be raised from
    sane places.

* Brief explanation of fixes

Stop allowing RCU_SOFTIRQ in idle

* How to test

In boot dmesg, the warning should be gone.

* What it could break.

N/A

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-bluefield/5.15.0-1012.14 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-jammy' to 'verification-done-jammy'. If the problem still exists, change the tag 'verification-needed-jammy' to 'verification-failed-jammy'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!