CVE-2026-29648: In OpenXiangShan NEMU, when Smstateen is enabled, clearing mstateen0

In OpenXiangShan NEMU, when Smstateen is enabled, clearing mstateen0.ENVCFG does not correctly restrict access to henvcfg and senvcfg. As a result, less-privileged code may read or write these CSRs without the required exception, potentially bypassing intended state-enable based isolation controls in virtualized or multi-privilege environments.