treewide: Replace GPLv2 boilerplate/reference with SPDX - gpl-2.0_8.RULE

Based on the normalized pattern:

    this program is free software you can redistribute it and/or modify it
    under the terms of the gnu general public license version 2 as
    published by the free software foundation

extracted by the scancode license scanner the SPDX license identifier

    GPL-2.0-only

has been chosen to replace the boilerplate/reference.

Reviewed-by: Allison Randal <allison@lohutok.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>