block: kill off q->flush_flags

Now that we converted everything to the newer block write cache
interface, kill off the queue flush_flags and queueable flush
entries.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 9531f5f..26f1497 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -1188,6 +1188,7 @@
 
 int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)
 {
+	struct request_queue *q = bdev_get_queue(rdev->bdev);
 	struct r5l_log *log;
 
 	if (PAGE_SIZE != 4096)
@@ -1197,7 +1198,7 @@
 		return -ENOMEM;
 	log->rdev = rdev;
 
-	log->need_cache_flush = (rdev->bdev->bd_disk->queue->flush_flags != 0);
+	log->need_cache_flush = test_bit(QUEUE_FLAG_WC, &q->queue_flags) != 0;
 
 	log->uuid_checksum = crc32c_le(~0, rdev->mddev->uuid,
 				       sizeof(rdev->mddev->uuid));