close_ioengine() clears ->data after calling engine cleanup

Then we can remove that bit from io engines.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/ioengines.c b/ioengines.c
index fb68763..662d55d 100644
--- a/ioengines.c
+++ b/ioengines.c
@@ -154,8 +154,10 @@
 {
 	dprint(FD_IO, "close ioengine %s\n", td->io_ops->name);
 
-	if (td->io_ops->cleanup)
+	if (td->io_ops->cleanup) {
 		td->io_ops->cleanup(td);
+		td->io_ops->data = NULL;
+	}
 
 	if (td->io_ops->dlhandle)
 		dlclose(td->io_ops->dlhandle);