treewide: Replace GPLv2 boilerplate/reference with SPDX - gpl-2.0_406.RULE

Based on the normalized pattern:

    this file is licensed under the term of the gnu general public license
    version 2 the program licensed as is without any warranty of any kind
    whether express or implied

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>