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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 89c1905 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
Signed-off-by: Steve Sistare <[email protected]>
Acked-by: Vivek Kasireddy <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Steve Sistare

mm/gup.c

@@ -3615,7 +3615,7 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
 	pgoff_t start_idx, end_idx, next_idx;
 	struct folio *folio = NULL;
 	struct folio_batch fbatch;
-	struct hstate *h;
+	struct hstate *h = NULL;
 	long ret = -EINVAL;
 
 	if (start < 0 || start > end || !max_folios)
@@ -3659,6 +3659,8 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
 							     &fbatch);
 			if (folio) {
 				folio_put(folio);
+				if (h)
+					folio_put(folio);
 				folio = NULL;
 			}