smp: Refactor remote CPU selection in smp_call_function_any()

smp_call_function_any() disables preemption across the entire operation:
selecting a target CPU, enqueueing the IPI, and synchronously waiting for
the remote CPU. smp_call_function_single() already re-enables preemption
before the synchronous csd_lock_wait(), so callers of
smp_call_function_any() should benefit from the same shorter
preemption-disabled section.

Simply removing get_cpu() and put_cpu() from smp_call_function_any()
would leave the preemption disablement entirely to
smp_call_function_single(). That opens a preemption window between
selecting the remote CPU, for example via sched_numa_find_nth_cpu(), and
dispatching the IPI in smp_call_function_single(). If the selected CPU is
fully offlined in that window, smp_call_function_single() fails its
cpu_online() check and returns -ENXIO to the caller, violating the
guarantee that smp_call_function_any() executes on any online CPU in the
mask.

Move the remote CPU selection into a common
__smp_call_function_single() helper. Keep the target CPU selection and
IPI dispatch within the same preemption-disabled region, while still
allowing the wait path to use the shorter preemption-disabled section
provided by smp_call_function_single().

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/20260709122933.4021501-4-zhouchuyi@bytedance.com

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