scftorture: Remove preempt_disable() in scftorture_invoke_one()

The smp_call*() functions handle their required preemption and CPU
pinning internally. The explicit preempt_disable() in
scftorture_invoke_one() is therefore no longer required for correctness.
Keeping the outer preempt_disable() would also prevent scftorture from
exercising the narrowed internal preemption-disabled regions during IPI
dispatch.

Removing the outer preemption protection can expose a CPU hotplug race in
the test validation when use_cpus_read_lock is false. For multicast
operations, SCF_PRIM_MANY or SCF_PRIM_ALL, if only one CPU is online,
smp_call_function_many() correctly skips sending IPIs and leaves scfc_out
false. Without preemption disabled, a CPU hotplug thread can preempt the
test thread, bring a second CPU online and increment num_online_cpus().
When the test thread resumes, the validation check can observe
num_online_cpus() > 1 and falsely trigger the memory-ordering warning,
leaking the scfcp structure.

Remove the preempt_disable() and preempt_enable() pairs around the
smp_call*() invocations in scftorture_invoke_one(). Restrict the
num_online_cpus() > 1 validation to the use_cpus_read_lock=true case,
where the CPU count is stable during the evaluation.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260709122933.4021501-10-zhouchuyi@bytedance.com

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