RISC-V: KVM: Write hgatp register with valid mode bits

In the Linux kernel, the following vulnerability has been resolved: RISC-V: KVM: Write hgatp register with valid mode bits According to the RISC-V Privileged Architecture Spec, when MODE=Bare is selected,software must write zero to the remaining fields of hgatp. We have detected the valid mode supported by the HW before, So using a valid mode to detect how many vmid bits are supported.