Address review comments

We should not enlarge the socket buffers when:
 - We are using a test dial (there are no buffers to enlarge).
 - A connection has been initialized with socket options which means
   that the user could have specified fixed buffer sizes.

conn.go

@@ -250,10 +250,10 @@ func (cc *Conn) Flush() error {
 	}
 	defer func() { _ = closer() }()
 
-	if err = cc.enlargeWriteBuffer(conn); err != nil {
+	if err := cc.enlargeWriteBuffer(conn); err != nil {
 		return err
 	}
-	if err = cc.enlargeReadBuffer(conn); err != nil {
+	if err := cc.enlargeReadBuffer(conn); err != nil {
 		return err
 	}
 
@@ -440,6 +440,22 @@ func (cc *Conn) getMessageSize() int {
 	return total
 }
 
+// canEnlargeBuffers returns true if the connection can automatically enlarge
+// the write and read buffers of the netlink connection.
+func (cc *Conn) canEnlargeBuffers() bool {
+	// If there are sock options, we assume that the user has already set the
+	// buffers to a fixed size.
+	if len(cc.sockOptions) > 0 {
+		return false
+	}
+
+	if cc.TestDial != nil {
+		return false
+	}
+
+	return true
+}
+
 // enlargeWriteBuffer automatically sets the write buffer of the given
 // connection to the accumulated message size. This is only done if the current
 // write buffer is smaller than the message size.
@@ -453,6 +469,10 @@ func (cc *Conn) getMessageSize() int {
 // TODO: Update this function to mimic the behavior of nftables once our
 // socket library supports multiple iovec entries.
 func (cc *Conn) enlargeWriteBuffer(conn *netlink.Conn) error {
+	if !cc.canEnlargeBuffers() {
+		return nil
+	}
+
 	messageSize := cc.getMessageSize()
 	writeBuffer, err := conn.WriteBuffer()
 	if err != nil {
@@ -481,6 +501,10 @@ func (cc *Conn) getDefaultEchoReadBuffer() int {
 //
 // See https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n426
 func (cc *Conn) enlargeReadBuffer(conn *netlink.Conn) error {
+	if !cc.canEnlargeBuffers() {
+		return nil
+	}
+
 	var bufferSize int
 
 	// If there are any messages with the Echo flag, we initialize the buffer size