vmscan: check folio_test_private(), not folio_get_private()

These two predicates are the same for file pages, but are not the same for
anonymous pages.

Link: https://lkml.kernel.org/r/20220902192639.1737108-3-willy@infradead.org
Fixes: 07f67a8d ("mm/vmscan: convert shrink_active_list() to use a folio")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -2550,8 +2550,8 @@ static void shrink_active_list(unsigned long nr_to_scan,
 		}
 
 		if (unlikely(buffer_heads_over_limit)) {
-			if (folio_get_private(folio) && folio_trylock(folio)) {
-				if (folio_get_private(folio))
+			if (folio_test_private(folio) && folio_trylock(folio)) {
+				if (folio_test_private(folio))
 					filemap_release_folio(folio, 0);
 				folio_unlock(folio);
 			}