mm/gup: fix memfd_pin_folios alloc race panic

If memfd_pin_folios tries to create a hugetlb page, but someone else
already did, then folio gets the value -EEXIST here:

        folio = memfd_alloc_folio(memfd, start_idx);
        if (IS_ERR(folio)) {
                ret = PTR_ERR(folio);
                if (ret != -EEXIST)
                        goto err;

then on the next trip through the "while start_idx" loop we panic here:

        if (folio) {
                folio_put(folio);

To fix, set the folio to NULL on error.

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

@@ -3702,6 +3702,7 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
 					ret = PTR_ERR(folio);
 					if (ret != -EEXIST)
 						goto err;
+					folio = NULL;
 				}
 			}
 		}