KVM: riscv: Fix Spectre-v1 in AIA CSR access

KVM: riscv: Fix Spectre-v1 in AIA CSR access

User-controlled indices are used to access AIA CSR registers.
Sanitize them with array_index_nospec() to prevent speculative
out-of-bounds access.

Similar to x86 commit 8c86405f606c ("KVM: x86: Protect
ioapic_read_indirect() from Spectre-v1/L1TF attacks") and arm64
commit 41b87599c743 ("KVM: arm/arm64: vgic: fix possible spectre-v1
in vgic_get_irq()").

Reviewed-by: Radim Krčmář <radim.krcmar@oss.qualcomm.com>
Signed-off-by: Lukas Gerlach <lukas.gerlach@cispa.de>
Link: https://lore.kernel.org/r/20260303-kvm-riscv-spectre-v1-v2-2-192caab8e0dc@cispa.de
Signed-off-by: Anup Patel <anup@brainfault.org>