block: fix blkdev_issue_flush() not detecting and passing EOPNOTSUPP back

This is important to eg dm, that tries to decide whether to stop using
barriers or not.

Tested as working by Anders Henke <anders.henke@1und1.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-barrier.c

@@ -259,8 +259,11 @@ int blk_do_ordered(struct request_queue *q, struct request **rqp)
 
 static void bio_end_empty_barrier(struct bio *bio, int err)
 {
-	if (err)
+	if (err) {
+		if (err == -EOPNOTSUPP)
+			set_bit(BIO_EOPNOTSUPP, &bio->bi_flags);
 		clear_bit(BIO_UPTODATE, &bio->bi_flags);
+	}
 
 	complete(bio->bi_private);
 }
@@ -309,7 +312,9 @@ int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
 		*error_sector = bio->bi_sector;
 
 	ret = 0;
-	if (!bio_flagged(bio, BIO_UPTODATE))
+	if (bio_flagged(bio, BIO_EOPNOTSUPP))
+		ret = -EOPNOTSUPP;
+	else if (!bio_flagged(bio, BIO_UPTODATE))
 		ret = -EIO;
 
 	bio_put(bio);