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>

mm/percpu.c

@@ -2143,9 +2143,9 @@ static void pcpu_reclaim_populated(void)
 	 * other accessor is the free path which only returns area back to the
 	 * allocator not touching the populated bitmap.
 	 */
-	while (!list_empty(&pcpu_chunk_lists[pcpu_to_depopulate_slot])) {
-		chunk = list_first_entry(&pcpu_chunk_lists[pcpu_to_depopulate_slot],
-					 struct pcpu_chunk, list);
+	while ((chunk = list_first_entry_or_null(
+			&pcpu_chunk_lists[pcpu_to_depopulate_slot],
+			struct pcpu_chunk, list))) {
 		WARN_ON(chunk->immutable);
 
 		/*