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>