LoongArch: mm: don't let GUP-fast follow invalidated THP PMDs

LoongArch 增加 pmd_access_permitted()，要求 _PAGE_PRESENT，使 GUP-fast 不再跟随 pmdp_invalidate() 留下 pmd_present() 仍真的失效 THP PMD，避免其拿 folio 引用破坏依赖引用计数的回收判断。