CVE-2025-21984 mm: fix kernel BUG when userfaultfd_move encounters swapcache

In the Linux kernel, the following vulnerability has been resolved: mm: fix kernel BUG when userfaultfdmove encounters swapcache userfaultfdmove checks whether the PTE entry is present or a swap entry. - If the PTE entry is present, movepresentpte handles folio migration by setting: srcfolio-inde...