Implement q_free to properly free queue elements

Previously, q_free() was defined but had no functionality.
Now it correctly frees all elements in the queue and releases
allocated memory.

Change-Id: Id0d688bc368e94281885f6e5fca0d69aefd417f3

Author: yy214123

queue.c

@@ -21,7 +21,21 @@ struct list_head *q_new()
 }
 
 /* Free all storage used by queue */
-void q_free(struct list_head *head) {}
+void q_free(struct list_head *head)
+{
+    if (!head)
+        return;
+    if (list_empty(head)) {
+        free(head);
+        return;
+    }
+    struct list_head *node, *safe;
+    list_for_each_safe (node, safe, head) {
+        element_t *current = list_entry(node, element_t, list);
+        q_release_element(current);
+    }
+    free(head);
+}
 
 /* Insert an element at head of queue */
 bool q_insert_head(struct list_head *head, char *s)