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>