mm/damon: minor cleanup for damon_pa_young

if need_lock is true but folio_trylock fails, we should return false
instead of NULL to match the return value type exactly. No functional
change intended.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

mm/damon/paddr.c

@@ -152,7 +152,7 @@ static bool damon_pa_young(unsigned long paddr, unsigned long *page_sz)
 	need_lock = !folio_test_anon(folio) || folio_test_ksm(folio);
 	if (need_lock && !folio_trylock(folio)) {
 		folio_put(folio);
-		return NULL;
+		return false;
 	}
 
 	rmap_walk(folio, &rwc);