Flush: fix error leak when flushing multiple messages

When you flush multiple messages/ops on a connection, and if flush fails
to apply, the netlink connection returns errors per command. Since we
are returning on noticing the first error, the rest of the errors are
buffered and leaks into the result of next flush.

This pull request invokes `conn.Receive()` * number of messages to drain
any buffered errors in the connection.

conn.go

@@ -230,16 +230,21 @@ func (cc *Conn) Flush() error {
 		return fmt.Errorf("SendMessages: %w", err)
 	}
 
+	var errs error
 	// Fetch the requested acknowledgement for each message we sent.
 	for _, msg := range cc.messages {
 		if msg.Header.Flags&netlink.Acknowledge == 0 {
 			continue // message did not request an acknowledgement
 		}
 		if _, err := conn.Receive(); err != nil {
-			return fmt.Errorf("conn.Receive: %w", err)
+			errs = errors.Join(errs, err)
 		}
 	}
 
+	if errs != nil {
+		return fmt.Errorf("conn.Receive: %w", errs)
+	}
+
 	return nil
 }