powerpc/preempt: Don't touch the idle task's preempt_count during hotplug

Powerpc currently resets a CPU's idle task preempt_count to 0 before
said task starts executing the secondary startup routine (and becomes an
idle task proper).

This conflicts with commit f1a0a376 ("sched/core: Initialize the
idle task with preemption disabled").

which initializes all of the idle tasks' preempt_count to
PREEMPT_DISABLED during smp_init(). Note that this was superfluous
before said commit, as back then the hotplug machinery would invoke
init_idle() via idle_thread_get(), which would have already reset the
CPU's idle task's preempt_count to PREEMPT_ENABLED.

Get rid of this preempt_count write.

Fixes: f1a0a376 ("sched/core: Initialize the idle task with preemption disabled")
Reported-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210707183831.2106509-1-valentin.schneider@arm.com

arch/powerpc/platforms/cell/smp.c

@@ -78,9 +78,6 @@ static inline int smp_startup_cpu(unsigned int lcpu)
 
 	pcpu = get_hard_smp_processor_id(lcpu);
 
-	/* Fixup atomic count: it exited inside IRQ handler. */
-	task_thread_info(paca_ptrs[lcpu]->__current)->preempt_count	= 0;
-
 	/*
 	 * If the RTAS start-cpu token does not exist then presume the
 	 * cpu is already spinning.

arch/powerpc/platforms/pseries/smp.c

@@ -105,9 +105,6 @@ static inline int smp_startup_cpu(unsigned int lcpu)
 		return 1;
 	}
 
-	/* Fixup atomic count: it exited inside IRQ handler. */
-	task_thread_info(paca_ptrs[lcpu]->__current)->preempt_count	= 0;
-
 	/* 
 	 * If the RTAS start-cpu token does not exist then presume the
 	 * cpu is already spinning.