null_blk: fix checking for REQ_FUA

null_handle_bio() erroneously uses the bio_op macro
which masks respective request flag bits including REQ_FUA
out thus failing the check.

Fix by checking bio->bi_opf directly.
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -1104,7 +1104,7 @@ static int null_handle_bio(struct nullb_cmd *cmd)
 		len = bvec.bv_len;
 		err = null_transfer(nullb, bvec.bv_page, len, bvec.bv_offset,
 				     op_is_write(bio_op(bio)), sector,
-				     bio_op(bio) & REQ_FUA);
+				     bio->bi_opf & REQ_FUA);
 		if (err) {
 			spin_unlock_irq(&nullb->lock);
 			return err;