RISC-V: KVM: fix off-by-one array access in SBI PMU

RISC-V: KVM: fix off-by-one array access in SBI PMU

The indexed array only has RISCV_KVM_MAX_COUNTERS elements.
The out-of-bound access could have been performed by a guest,
but it could only access another guest accessible data.

Fixes: 8f0153ecd3bf ("RISC-V: KVM: Add skeleton support for perf")
Signed-off-by: Radim Krčmář <radim.krcmar@oss.qualcomm.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260227134617.23378-1-radim.krcmar@oss.qualcomm.com
Signed-off-by: Anup Patel <anup@brainfault.org>