RISC-V: KVM: Reject overlapping AIA IMSIC address fields

KVM lets userspace configure the IMSIC guest, HART, group-width, and
group-shift fields independently. The individual bounds allow a layout
such as guest_bits=7, hart_bits=6, group_bits=1, and group_shift=24.

That layout places the group index bit at bit 24 while the lower
guest+HART field also extends through bit 24. The IMSIC address helpers
later consume these values as one address layout: aia_imsic_ppn() masks
the lower guest+HART field and the group field, while
aia_imsic_hart_index() combines the group and HART index. Allowing an
overlapping configuration therefore gives one address bit two different
meanings.

Reject group-index layouts that start before the end of the guest+HART
field before initializing the in-kernel irqchip.

Fixes: 89d01306e34d ("RISC-V: KVM: Implement device interface for AIA irqchip")
Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260718040000.005.6ede2881-caplitmus-rv@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

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