netlink: Use netlink header as base to calculate bad attribute offset

Userspace might send a batch that is composed of several netlink
messages. The netlink_ack() function must use the pointer to the netlink
header as base to calculate the bad attribute offset.

Fixes: 2d4bc933 ("netlink: extended ACK reporting")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -2434,7 +2434,7 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 							       in_skb->len))
 				WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
 						    (u8 *)extack->bad_attr -
-						    in_skb->data));
+						    (u8 *)nlh));
 		} else {
 			if (extack->cookie_len)
 				WARN_ON(nla_put(skb, NLMSGERR_ATTR_COOKIE,