security,rcu: Convert call_rcu(whitelist_item_free) to kfree_rcu()

The rcu callback whitelist_item_free() just calls a kfree(),
so we use kfree_rcu() instead of the call_rcu(whitelist_item_free).

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: James Morris <jmorris@namei.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>