smp/hotplug: Replace BUG_ON and react useful

The move of the unpark functions to the control thread moved the BUG_ON()
there as well. While it made some sense in the idle thread of the upcoming
CPU, it's bogus to crash the control thread on the already online CPU,
especially as the function has a return value and the callsite is prepared
to handle an error return.

Replace it with a WARN_ON_ONCE() and return a proper error code.

Fixes: 9cd4f1a4e7a8 ("smp/hotplug: Move unparking of percpu threads to the control CPU")
Rightfully-ranted-at-by: Linux Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/cpu.c

@@ -279,7 +279,8 @@ static int bringup_wait_for_ap(unsigned int cpu)
 
 	/* Wait for the CPU to reach CPUHP_AP_ONLINE_IDLE */
 	wait_for_completion(&st->done);
-	BUG_ON(!cpu_online(cpu));
+	if (WARN_ON_ONCE((!cpu_online(cpu))))
+		return -ECANCELED;
 
 	/* Unpark the stopper thread and the hotplug thread of the target cpu */
 	stop_machine_unpark(cpu);