rootfs-handoff: implement remount,rw after filesystem verification

rootfs-handoff: implement remount,rw after filesystem verification

- Implemented remount,rw after successful filesystem verification
- Uses vfs_mount_full with MS_REMOUNT flag to update mount flags
- Remounts filesystem read-write if it was forced read-only due to verification
- If remount fails, filesystem remains read-only with warning message
- This completes the 'root verification and remount,rw after checks' item from ROOTFS_HANDOFF_PLAN.md

Test Results:
- Smoke test: 131/131 PASS
- Battery test: 157/157 PASS
- Analyzer: PASS
- Zero regressions