writeback: fix the wrong congested state variable definition

The right variable definition should be wb_congested_state that
include WB_async_congested and WB_sync_congested. So fix it.
Signed-off-by: Kaixu Xia <xiakaixu@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

mm/backing-dev.c

@@ -898,7 +898,7 @@ static atomic_t nr_wb_congested[2];
 void clear_wb_congested(struct bdi_writeback_congested *congested, int sync)
 {
 	wait_queue_head_t *wqh = &congestion_wqh[sync];
-	enum wb_state bit;
+	enum wb_congested_state bit;
 
 	bit = sync ? WB_sync_congested : WB_async_congested;
 	if (test_and_clear_bit(bit, &congested->state))
@@ -911,7 +911,7 @@ EXPORT_SYMBOL(clear_wb_congested);
 
 void set_wb_congested(struct bdi_writeback_congested *congested, int sync)
 {
-	enum wb_state bit;
+	enum wb_congested_state bit;
 
 	bit = sync ? WB_sync_congested : WB_async_congested;
 	if (!test_and_set_bit(bit, &congested->state))