slsm: grab the tree mutex while serializing

slsm_serialize_subtree allocates, so we can't rely on RCU.

security/slsm/tree.c

@@ -263,7 +263,6 @@ ssize_t slsm_serialize_subtree(struct slsm_str *buf, struct slsm_str *path_buf,
 }
 
 ssize_t slsm_serialize_tree(char **retbuf) {
-	struct tree_node *t;
 	ssize_t ret;
 	struct slsm_str buf;
 	struct slsm_str path_buf;
@@ -278,10 +277,9 @@ ssize_t slsm_serialize_tree(char **retbuf) {
 	if (ret)
 		return ret;
 
-	rcu_read_lock();
-	t = rcu_dereference(rules);
-	ret = slsm_serialize_subtree(&buf, &path_buf, t);
-	rcu_read_unlock();
+	mutex_lock(&mutex);
+	ret = slsm_serialize_subtree(&buf, &path_buf, rules);
+	mutex_unlock(&mutex);
 
 	if (ret)
 		return ret;