selinux: various sparse fixes

When running the SELinux code through sparse, there are a handful of
warnings.  This patch resolves some of these warnings caused by
"__rcu" mismatches.

 % make W=1 C=1 security/selinux/

Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/netnode.c

@@ -164,8 +164,9 @@ static void sel_netnode_insert(struct sel_netnode *node)
 	if (sel_netnode_hash[idx].size == SEL_NETNODE_HASH_BKT_LIMIT) {
 		struct sel_netnode *tail;
 		tail = list_entry(
-			rcu_dereference_protected(sel_netnode_hash[idx].list.prev,
-						  lockdep_is_held(&sel_netnode_lock)),
+			rcu_dereference_protected(
+				list_tail_rcu(&sel_netnode_hash[idx].list),
+				lockdep_is_held(&sel_netnode_lock)),
 			struct sel_netnode, list);
 		list_del_rcu(&tail->list);
 		kfree_rcu(tail, rcu);