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>