riscv: kvm: return SBI_ERR_FAILURE for pmu_snapshot_set_shmem() when OOM

riscv: kvm: return SBI_ERR_FAILURE for pmu_snapshot_set_shmem() when OOM

kvm_riscv_vcpu_pmu_snapshot_set_shmem() returned -ENOMEM from the
SBI extension handler, which caused kvm_riscv_vcpu_sbi_ecall() to
abort KVM_RUN and surface the error to userspace instead of
ompleting the ECALL with a negative SBI error in a0.
Use SBI_ERR_FAILURE and the normal retdata path, matching other PMU
handlers and kvm_sbi_ext_pmu_handler comment.

Fixes: c2f41ddbcdd7 ("RISC-V: KVM: Implement SBI PMU Snapshot feature")
Cc: stable@vger.kernel.org
Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260514173642.41448-1-osama.abdelkader@gmail.com
Signed-off-by: Anup Patel <anup@brainfault.org>