netfilter: nfnetlink: Handle ACK flags for batch messages

The NLM_F_ACK flag is ignored for nfnetlink batch begin and end
messages. This is a problem for ynl which wants to receive an ack for
every message it sends, not just the commands in between the begin/end
messages.

Add processing for ACKs for begin/end messages and provide responses
when requested.

I have checked that iproute2, pyroute2 and systemd are unaffected by
this change since none of them use NLM_F_ACK for batch begin/end.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240418104737.77914-5-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/netfilter/nfnetlink.c

@@ -427,6 +427,9 @@ static void nfnetlink_rcv_batch(struct sk_buff *skb, struct nlmsghdr *nlh,
 
 	nfnl_unlock(subsys_id);
 
+	if (nlh->nlmsg_flags & NLM_F_ACK)
+		nfnl_err_add(&err_list, nlh, 0, &extack);
+
 	while (skb->len >= nlmsg_total_size(0)) {
 		int msglen, type;
 
@@ -573,6 +576,8 @@ static void nfnetlink_rcv_batch(struct sk_buff *skb, struct nlmsghdr *nlh,
 		} else if (err) {
 			ss->abort(net, oskb, NFNL_ABORT_NONE);
 			netlink_ack(oskb, nlmsg_hdr(oskb), err, NULL);
+		} else if (nlh->nlmsg_flags & NLM_F_ACK) {
+			nfnl_err_add(&err_list, nlh, 0, &extack);
 		}
 	} else {
 		enum nfnl_abort_action abort_action;