x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()

x86 effective_prot() 引入 mm_first_pgtable_level() 并识别 mm_pXd_folded()，仅在非折叠且非首层时更新 higher_prot，避免 ptdump 对折叠项取值误判。