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>