homework: explicitly close cryptsetup context, to not keep loopback device busy

The cryptsetup context pins the loop device even after deactivation.
Let's explicitly release the context to make sure the subsequent
loopback device detaching works cleanly.

src/home/homework-luks.c

@@ -2166,6 +2166,9 @@ int home_create_luks(
                 goto fail;
         }
 
+        crypt_free(cd);
+        cd = NULL;
+
         dm_activated = false;
 
         loop = loop_device_unref(loop);