rcu: Remove redundant return from rcu_report_exp_rnp()

Empty void functions do not need "return", so this commit removes it
from rcu_report_exp_rnp().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>