RISC-V: KVM: Fix array out-of-bounds in pmu_ctr_read() and pmu_fw_ctr_read_hi()

When a guest invokes SBI_EXT_PMU_COUNTER_FW_READ or
SBI_EXT_PMU_COUNTER_FW_READ_HI on a firmware counter that has not been
configured via SBI_EXT_PMU_COUNTER_CFG_MATCH, the pmc->event_idx remains
SBI_PMU_EVENT_IDX_INVALID (0xFFFFFFFF). get_event_code() extracts the
lower 16 bits, yielding 0xFFFF (65535), which is then used to index into
kvpmu->fw_event[]. Since fw_event is only RISCV_KVM_MAX_FW_CTRS (32)
entries, this triggers an array-index-out-of-bounds:

  UBSAN: array-index-out-of-bounds in arch/riscv/kvm/vcpu_pmu.c:255:37
  index 65535 is out of range for type 'kvm_fw_event [32]'

Add a check for the known unconfigured case (SBI_PMU_EVENT_IDX_INVALID)
and a WARN_ONCE guard for any unexpected out-of-bounds event codes,
returning -EINVAL in both cases.

Fixes: badc386869e2c ("RISC-V: KVM: Support firmware events")
Fixes: 08fb07d6dcf71 ("RISC-V: KVM: Support 64 bit firmware counters on RV32")
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Reviewed-by: Andrew Jones <andrew.jones@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260316014533.2312254-2-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

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