genetlink: fix possible memory leak in genl_family_rcv_msg()

'attrbuf' is malloced in genl_family_rcv_msg() when family->maxattr &&
family->parallel_ops, thus should be freed before leaving from the error
handling cases, otherwise it will cause memory leak.

Introduced by commit def3117493eafd9dfa1f809d861e0031b2cc8a07
(genl: Allow concurrent genl callbacks.)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/genetlink.c

@@ -598,7 +598,7 @@ static int genl_family_rcv_msg(struct genl_family *family,
 		err = nlmsg_parse(nlh, hdrlen, attrbuf, family->maxattr,
 				  ops->policy);
 		if (err < 0)
-			return err;
+			goto out;
 	}
 
 	info.snd_seq = nlh->nlmsg_seq;
@@ -613,7 +613,7 @@ static int genl_family_rcv_msg(struct genl_family *family,
 	if (family->pre_doit) {
 		err = family->pre_doit(ops, skb, &info);
 		if (err)
-			return err;
+			goto out;
 	}
 
 	err = ops->doit(skb, &info);
@@ -621,6 +621,7 @@ static int genl_family_rcv_msg(struct genl_family *family,
 	if (family->post_doit)
 		family->post_doit(ops, skb, &info);
 
+out:
 	if (family->parallel_ops)
 		kfree(attrbuf);