mm/hugetlb: fix memfd_pin_folios free_huge_pages leak

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

mm/hugetlb: fix memfd_pin_folios free_huge_pages leak

memfd_pin_folios followed by unpin_folios fails to restore free_huge_pages

if the pages were not already faulted in, because the folio refcount for

pages created by memfd_alloc_folio never goes to 0. memfd_pin_folios

needs another folio_put to undo the folio_try_get below:

memfd_alloc_folio()

alloc_hugetlb_folio_nodemask()

dequeue_hugetlb_folio_nodemask()

dequeue_hugetlb_folio_node_exact()

folio_ref_unfreeze(folio, 1); ; adds 1 refcount

folio_try_get() ; adds 1 refcount

hugetlb_add_to_page_cache() ; adds 512 refcount (on x86)

With the fix, after memfd_pin_folios + unpin_folios, the refcount for the

(unfaulted) page is 512, which is correct, as the refcount for a faulted

unpinned page is 513.