RISC-V: KVM: Document a TOCTOU race in SBI system suspend handler

The SUSP handler checks that all other vCPUs are stopped before
entering system suspend, but a concurrent HSM HART_START can start
a vCPU after it has already passed the check.

This is a known TOCTOU race. We do not fix it because:
1. Triggering it requires a pathological guest.
2. Only guest state is at risk, not host integrity.
3. Userspace can double-check vCPU states before suspend.

Add a comment documenting the race and the rationale for not fixing it.

Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Assisted-by: YuanSheng:DeepSeek-V3.2
Reviewed-by: Andrew Jones <andrew.jones@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260525013642.999187-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

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