siphash: add SPDX tags as sole licensing authority

The text "dual BSD/GPLv2 license" is somewhat ambiguous, and moving this
over to SPDX is overdue. This commit adds SPDX tags to the relevant
files and clarifies that it's GPLv2 only and 3-clause BSD. It also
removes the old text, so that the SPDX tags are the only source of the
information.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/siphash.h

-/* Copyright (C) 2016 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
- *
- * This file is provided under a dual BSD/GPLv2 license.
+/* SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause) */
+/* Copyright (C) 2016-2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
  *
  * SipHash: a fast short-input PRF
  * https://131002.net/siphash/

lib/siphash.c

-/* Copyright (C) 2016 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
- *
- * This file is provided under a dual BSD/GPLv2 license.
+// SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause)
+/* Copyright (C) 2016-2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
  *
  * SipHash: a fast short-input PRF
  * https://131002.net/siphash/

lib/test_siphash.c

-/* Test cases for siphash.c
+// SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause)
+/* Copyright (C) 2016-2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
  *
- * Copyright (C) 2016 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
- *
- * This file is provided under a dual BSD/GPLv2 license.
+ * Test cases for siphash.c
  *
  * SipHash: a fast short-input PRF
  * https://131002.net/siphash/