linux/rcu_sync: Convert to SPDX license identifier

Replace the license boiler plate with a SPDX license identifier.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
[ paulmck: Update .h SPDX format per Joe Perches. ]
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/rcu_sync.h

+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * RCU-based infrastructure for lightweight reader-writer locking
  *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 2 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, you can access it online at
- * http://www.gnu.org/licenses/gpl-2.0.html.
- *
  * Copyright (c) 2015, Red Hat, Inc.
  *
  * Author: Oleg Nesterov <oleg@redhat.com>