kvfree_rcu: Use kfree_rcu_monitor() instead of open-coded variant

Replace an open-coded version of the kfree_rcu_monitor() function body
with a call to that function.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>