Implement q_free

Using list_for_each_entry_safe to enumerate every node in the
queue, then using q_release_element to free the choosing node.
Finally free the head.

This commit ensures that all elements are freed for avoiding
memory leak.

Change-Id: I434a7d16dab3e8627c6c0016ff641520067a47d9

Author: Mike1117

queue.c

@@ -22,7 +22,16 @@ 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;
+    element_t *cur, *safe;
+    /* cppcheck-suppress unknownMacro */
+    list_for_each_entry_safe (cur, safe, head, list)
+        q_release_element(cur);
+    free(head);
+}
 
 /* Insert an element at head of queue */
 bool q_insert_head(struct list_head *head, char *s)