treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 53

Based on 1 normalized pattern(s):

  this code may be copied under the gpl v 2 or at your option any
  later version

extracted by the scancode license scanner the SPDX license identifier

  GPL-2.0-or-later

has been chosen to replace the boilerplate/reference in 1 file(s).
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Richard Fontana <rfontana@redhat.com>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Allison Randal <allison@lohutok.net>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190520071858.029737698@linutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/decnet/netfilter/dn_rtmsg.c

+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * DECnet       An implementation of the DECnet protocol suite for the LINUX
  *              operating system.  DECnet is implemented using the  BSD Socket
@@ -6,11 +7,8 @@
  *              DECnet Routing Message Grabulator
  *
  *              (C) 2000 ChyGwyn Limited  -  http://www.chygwyn.com/
- *              This code may be copied under the GPL v.2 or at your option
- *              any later version.
  *
  * Author:      Steven Whitehouse <steve@chygwyn.com>
- *
  */
 #include <linux/module.h>
 #include <linux/skbuff.h>