Fix operator precedence to apply default value

Related-Prod: PRODX-56774
Change-Id: Id41e40f414e69a2c4bc379189509fc168499cd87
diff --git a/manila_tempest_tests/tests/scenario/test_share_extend.py b/manila_tempest_tests/tests/scenario/test_share_extend.py
index 39098dc..8ba674b 100644
--- a/manila_tempest_tests/tests/scenario/test_share_extend.py
+++ b/manila_tempest_tests/tests/scenario/test_share_extend.py
@@ -87,7 +87,8 @@
         # Additional blocks that exceed the share capacity, defaulting to 5.
         overflow_blocks = (
             total_blocks - three_quarter_blocks +
-            additional_overflow_blocks or 5)
+            (additional_overflow_blocks or 5)
+        )
         LOG.debug('Step 6b - Write more data, should fail')
         self.assertRaises(
             exceptions.SSHExecCommandFailed,