btrfs: check if the device is flush capable

The block layer call chain from submit_bio will check if the write cache
is enabled for the given queue before submitting the flush. This will
add a code to fail fast if its not.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ updated changelog to reflect current code stat, blkdev_issue_flush is
  not used yet ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3518,9 +3518,13 @@ static void btrfs_end_empty_barrier(struct bio *bio)
  */
 static int write_dev_flush(struct btrfs_device *device, int wait)
 {
+	struct request_queue *q = bdev_get_queue(device->bdev);
 	struct bio *bio;
 	int ret = 0;
 
+	if (!test_bit(QUEUE_FLAG_WC, &q->queue_flags))
+		return 0;
+
 	if (wait) {
 		bio = device->flush_bio;
 		if (!bio)