powerpc/smp: do not decrement idle task preempt count in CPU offline

In the Linux kernel, the following vulnerability has been resolved:

powerpc/smp: do not decrement idle task preempt count in CPU offline

With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we

get:

BUG: scheduling while atomic: swapper/1/0/0x00000000

no locks held by swapper/1/0.

CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100

Call Trace:

dump_stack_lvl+0xac/0x108

__schedule_bug+0xac/0xe0

__schedule+0xcf8/0x10d0

schedule_idle+0x3c/0x70

do_idle+0x2d8/0x4a0

cpu_startup_entry+0x38/0x40

start_secondary+0x2ec/0x3a0

start_secondary_prolog+0x10/0x14

This is because powerpc's arch_cpu_idle_dead() decrements the idle task's

preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc:

Re-enable preemption before cpu_die()"), specifically "start_secondary()

expects a preempt_count() of 0."

However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle

task's preempt_count during hotplug") and commit f1a0a376ca0c ("sched/core:

Initialize the idle task with preemption disabled"), that justification no

longer holds.

The idle task isn't supposed to re-enable preemption, so remove the

vestigial preempt_enable() from the CPU offline path.

Tested with pseries and powernv in qemu, and pseries on PowerVM.