[copyright][license] Switch to yearless copyright and some cleanup in the linux-kernel files

* Switch to yearless copyright per FB policy
* Fix up SPDX-License-Identifier lines in `contrib/linux-kernel` sources
* Add zstd copyright/license header to the `contrib/linux-kernel` sources
* Update the `tests/test-license.py` to check for yearless copyright
* Improvements to `tests/test-license.py`
* Check `contrib/linux-kernel` in `tests/test-license.py`

lib/common/xxhash.c

@@ -1,6 +1,6 @@
 /*
  *  xxHash - Fast Hash algorithm
- *  Copyright (c) 2012-2021, Yann Collet, Facebook, Inc.
+ *  Copyright (c) Yann Collet, Facebook, Inc.
  *
  *  You can contact the author at :
  *  - xxHash homepage: http://www.xxhash.com