shared: add c_list_length_is() helper

shared/nm-utils/c-list-util.h

@@ -40,4 +40,27 @@ void c_list_sort (CList *head,
                   CListSortCmp cmp,
                   const void *user_data);
 
+/* c_list_length_is:
+ * @list: the #CList list head
+ * @check_len: the length to compare
+ *
+ * Returns: basically the same as (c_list_length (@list) == @check_len),
+ *   but does not require to iterate the entire list first. There is only
+ *   one real use: to find out whether there is exactly one element in the
+ *   list, by passing @check_len as 1.
+ */
+static inline int
+c_list_length_is (const CList *list, unsigned long check_len) {
+	unsigned long n = 0;
+	const CList *iter;
+
+	c_list_for_each (iter, list) {
+		++n;
+		if (n > check_len)
+			return 0;
+	}
+
+	return n == check_len;
+}
+
 #endif /* __C_LIST_UTIL_H__ */