Use loop_control.loop_var directly

6eefc11c converted task.loop_control into an object, but while the other
callers were updated to use .loop_var instead of .get('loop_var'), this
site was overlooked.

This can be reproduced by including with loop_control a file that does
set_fact; a simple regression test along these lines is included.

test/integration/test_includes3.yml

@@ -0,0 +1,7 @@
+---
+- hosts: localhost
+  tasks:
+    - include: test_includes4.yml
+      with_items: ["a"]
+      loop_control:
+        loop_var: r