Implement q_size function

Implement q_size() to get the size of the queue.

queue.c

@@ -47,7 +47,14 @@ element_t *q_remove_tail(struct list_head *head, char *sp, size_t bufsize)
 /* Return number of elements in queue */
 int q_size(struct list_head *head)
 {
-    return -1;
+    if (!head)
+        return 0;
+
+    int size = 0;
+    struct list_head *curr;
+    list_for_each (curr, head)
+        size++;
+    return size;
 }
 
 /* Delete the middle node in queue */