mm/khugepaged: Never install PMDs in uffd-minor-registered VMAs

MADV_COLLAPSE 会在 uffd-minor VMA 的洞上装 PMD，导致 UFFDIO_CONTINUE 返回 EEXIST；为避免 process_madvise 外部干扰 resolver，禁止在 uffd-minor VMA 安装 PMD，只撤 PTE。