treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 431

Based on 1 normalized pattern(s):

  as should be obvious for linux kernel code license is gplv2

extracted by the scancode license scanner the SPDX license identifier

  GPL-2.0-only

has been chosen to replace the boilerplate/reference in 1 file(s).
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Allison Randal <allison@lohutok.net>
Reviewed-by: Armijn Hemel <armijn@tjaldur.nl>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190531190114.565827206@linutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/btree.c

+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * lib/btree.c	- Simple In-memory B+Tree
  *
- * As should be obvious for Linux kernel code, license is GPLv2
- *
  * Copyright (c) 2007-2008 Joern Engel <joern@purestorage.com>
  * Bits and pieces stolen from Peter Zijlstra's code, which is
  * Copyright 2007, Red Hat Inc. Peter Zijlstra
- * GPLv2
  *
  * see http://programming.kicks-ass.net/kernel-patches/vma_lookup/btree.patch
  *