mm/memfd_luo: fix physical address conversion in put_folios cleanup

In the Linux kernel, the following vulnerability has been resolved:

mm/memfd_luo: fix physical address conversion in put_folios cleanup

In memfd_luo_retrieve_folios()'s put_folios cleanup path:

1. kho_restore_folio() expects a phys_addr_t (physical address) but
receives a raw PFN (pfolio->pfn). This causes kho_restore_page() to
check the wrong physical address (pfn << PAGE_SHIFT instead of the
actual physical address).

2. This loop lacks the !pfolio->pfn check that exists in the main
retrieval loop and memfd_luo_discard_folios(), which could
incorrectly process sparse file holes where pfn=0.

Fix by converting PFN to physical address with PFN_PHYS() and adding
the !pfolio->pfn check, matching the pattern used elsewhere in this file.

This issue was identified by the AI review.
https://sashiko.dev/#/patchset/20260323110747.193569-1-duanchenghao@kylinos.cn