security,rcu: Convert call_rcu(sel_netnode_free) to kfree_rcu()
The rcu callback sel_netnode_free() just calls a kfree(),
so we use kfree_rcu() instead of the call_rcu(sel_netnode_free).
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Stephen Smalley <sds@tycho.nsa.gov> Cc: James Morris <jmorris@namei.org> Cc: Eric Paris <eparis@parisplace.org> Reviewed-by: Josh Triplett <josh@joshtriplett.org>