secretmem: remove uses of struct page

secretmem: remove uses of struct page

Use filemap_lock_folio() instead of find_lock_page() to retrieve
a folio from the page cache.

[lorenzo.stoakes@oracle.com: fix check of filemap_lock_folio() return value]
Link: https://lkml.kernel.org/r/fdbca1d0-01a3-4653-85ed-cf257bb848be@lucifer.local
Link: https://lkml.kernel.org/r/20250613194744.3175157-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>