linux/rcu_node_tree: Convert to SPDX license identifier

Replace the license boiler plate with a SPDX license identifier.
While in the area, update an email address.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
[ paulmck: Update .h SPDX comment format per Joe Perches. ]
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
1 file changed