LoongArch: KVM: Check validity of "num_cpu" from user space

The maximum supported cpu number is EIOINTC_ROUTE_MAX_VCPUS about
irqchip EIOINTC, here add validation about cpu number to avoid array
pointer overflow.

Cc: stable@vger.kernel.org
Fixes: 1ad7efa5 ("LoongArch: KVM: Add EIOINTC user mode read and write functions")
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>