s390: preempt: Fix preempt_count initialization

S390's init_idle_preempt_count(p, cpu) doesn't actually let us initialize the
preempt_count of the requested CPU's idle task: it unconditionally writes
to the current CPU's. This clearly conflicts with idle_threads_init(),
which intends to initialize *all* the idle tasks, including their
preempt_count (or their CPU's, if the arch uses a per-CPU preempt_count).

Unfortunately, it seems the way s390 does things doesn't let us initialize
every possible CPU's preempt_count early on, as the pages where this
resides are only allocated when a CPU is brought up and are freed when it
is brought down.

Let the arch-specific code set a CPU's preempt_count when its lowcore is
allocated, and turn init_idle_preempt_count() into an empty stub.

Fixes: f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
Reported-by: Guenter Roeck <[email protected]>
Signed-off-by: Valentin Schneider <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Reviewed-by: Heiko Carstens <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Vasily Gorbik <[email protected]>

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