smp-urgent-2022-04-17

 - Make the warning condition in flush_smp_call_function_queue() correct,
   which checks a just emptied list head for being empty instead of
   validating that there was no pending entry on the offlined CPU at all.

 - The @cpu member of struct cpuhp_cpu_state is initialized when the CPU
   hotplug thread for the upcoming CPU is created. That's too late because
   the creation of the thread can fail and then the following rollback
   operates on CPU0. Get rid of the CPU member and hand the CPU number to
   the involved functions directly.