Expose accumulated message size

AFAIU, netlink has a message size limit of ~32KB.

This means that a single transaction would be limited to a few
operations. Going over the limit would result in the following error
after flushing:

netlink receive: recvmsg: no buffer space available

If I am not mistaken, the only alternative is to then divide big
transactions into smaller chunks. The only issue is that it's hard
to tell when the limit has been exceeded.

That's why I suggest exposing a function that would produce the
accumulated message size since the messages slice is private.

- https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/netlink/af_netlink.c?id=1e26c5e28ca5821a824e90dd359556f5e9e7b89f#n1930
- https://web.git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=d35c99ff77ecb2eb239731b799386f3b3637a31e

conn.go

@@ -329,6 +329,22 @@ func (cc *Conn) FlushRuleset() {
 	})
 }
 
+// GetMessageSize returns the total size of all messages in the buffer.
+// This is useful for making sure that the messages will not exceed the limits
+// of the netlink buffer which is 32768 + ~320 bytes. See also
+// https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/netlink/af_netlink.c?id=1e26c5e28ca5821a824e90dd359556f5e9e7b89f#n1930
+// and
+// https://web.git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=d35c99ff77ecb2eb239731b799386f3b3637a31e
+func (cc *Conn) GetMessageSize() uint32 {
+	cc.mu.Lock()
+	defer cc.mu.Unlock()
+	var total uint32
+	for _, msg := range cc.messages {
+		total += uint32(len(msg.Data)) + 16 // 16 bytes for the header
+	}
+	return total
+}
+
 func (cc *Conn) dialNetlink() (*netlink.Conn, error) {
 	var (
 		conn *netlink.Conn