Set rule handle during flush

This change makes it possible to delete rules after inserting them,
without needing to query the rules first. Additionally, this allows
positioning a new rule next to an existing rule.

There are two ways to refer to a rule: Either by ID or by handle. The ID
is assigned by userspace, and is only valid within a transaction, so it
can only be used before the flush. The handle is assigned by the kernel
when the transaction is committed, and can thus only be used after the
flush. We thus need to set an ID on each newly created rule, and
retrieve the handle of the rule during the flush.

I implemented a new mechanism for retrieving replies in Flush, and
handling these replies by adding a callback to netlink messages. There
was some existing code to handle "overrun", which I deleted, because it
was nonsensical and just worked by accident. NLMSG_OVERRUN is in fact
not a flag, but a complete message type, so the (re&netlink.Overrun)
masking makes no sense. Even better, NLMSG_OVERRUN is never actually
used by Linux. What this code was actually doing was skipping over the
NFT_MSG_NEWRULE replies, and possibly a NFT_MSG_NEWGEN reply.

I updated tests to generate replies for the NFT_MSG_NEWRULE messages
with a handle added.

conn.go

@@ -52,7 +52,7 @@ type Conn struct {
 type netlinkMessage struct {
 	Header      netlink.Header
 	Data        []byte
-	rule   *Rule
+	handleReply func(reply netlink.Message) error
 }
 
 // ConnOption is an option to change the behavior of the nftables Conn returned by Open.
@@ -274,8 +274,7 @@ func (cc *Conn) Flush() error {
 		} else if replyIndex < len(cc.messages) {
 			msg := messages[replyIndex+1]
 			if msg.Header.Sequence == reply.Header.Sequence && msg.Header.Type == reply.Header.Type {
-				// The only messages which set the echo flag are rule create messages.
+				err := cc.messages[replyIndex].handleReply(reply)
-				err := cc.messages[replyIndex].rule.handleCreateReply(reply)
 				if err != nil {
 					errs = errors.Join(errs, err)
 				}

rule.go

@@ -174,14 +174,14 @@ func (cc *Conn) newRule(r *Rule, op ruleOperation) *Rule {
 	}
 
 	var flags netlink.HeaderFlags
-	var ruleRef *Rule
+	var handleReply func(reply netlink.Message) error
 	switch op {
 	case operationAdd:
 		flags = netlink.Request | netlink.Acknowledge | netlink.Create | netlink.Echo | netlink.Append
-		ruleRef = r
+		handleReply = r.handleCreateReply
 	case operationInsert:
 		flags = netlink.Request | netlink.Acknowledge | netlink.Create | netlink.Echo
-		ruleRef = r
+		handleReply = r.handleCreateReply
 	case operationReplace:
 		flags = netlink.Request | netlink.Acknowledge | netlink.Replace
 	}
@@ -202,7 +202,7 @@ func (cc *Conn) newRule(r *Rule, op ruleOperation) *Rule {
 			Flags: flags,
 		},
 		Data:        append(extraHeader(uint8(r.Table.Family), 0), msgData...),
-		rule: ruleRef,
+		handleReply: handleReply,
 	})
 
 	return r