mm/percpu: use list_first_entry_or_null in pcpu_reclaim_populated()

To replace list_empty()/list_first_entry() pair to simplify code.

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Dennis Zhou <dennis@kernel.org>