Make busy rx/tx loop faster.

This commit improves the performance of the rx and tx loop by 1) moving
allocations to outside the loop and by 2) eliminating an unnecessary
Write call.

As long as gvproxy implements the same improvements, this should result
in ~20% more reqs/sec.

For posterity, here's the upstream PR for the gvproxy side of things:
containers/gvisor-tap-vsock#215

proxy.go

@@ -12,7 +12,6 @@ import (
 	"time"
 
 	"github.com/containers/gvisor-tap-vsock/pkg/transport"
-	"github.com/songgao/packets/ethernet"
 	"github.com/songgao/water"
 	"github.com/vishvananda/netlink"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
@@ -122,39 +121,32 @@ func linkUp() error {
 	return netlink.LinkSetUp(link)
 }
 
-func rx(conn net.Conn, tap *water.Interface, errCh chan error, mtu int) {
+func rx(conn io.ReadWriteCloser, tap io.ReadWriteCloser, errCh chan error, mtu int) {
 	elog.Println("Waiting for frames from enclave application.")
-	var frame ethernet.Frame
+	frame := make([]byte, mtu)
+	size := make([]byte, 2)
 	for {
-		frame.Resize(mtu)
 		n, err := tap.Read([]byte(frame))
 		if err != nil {
 			errCh <- fmt.Errorf("failed to read packet from TAP device: %w", err)
 			return
 		}
-		frame = frame[:n]
 
-		size := make([]byte, 2)
 		binary.LittleEndian.PutUint16(size, uint16(n))
-
-		if _, err := conn.Write(size); err != nil {
-			errCh <- fmt.Errorf("failed to write frame size to connection: %w", err)
-			return
-		}
-		if _, err := conn.Write(frame); err != nil {
+		if _, err := conn.Write(append(size, frame[:n]...)); err != nil {
 			errCh <- fmt.Errorf("failed to write frame to connection: %w", err)
 			return
 		}
 	}
 }
 
-func tx(conn net.Conn, tap *water.Interface, errCh chan error, mtu int) {
+func tx(conn net.Conn, tap io.ReadWriteCloser, errCh chan error, mtu int) {
 	elog.Println("Waiting for frames from host.")
-	sizeBuf := make([]byte, 2)
+	size := make([]byte, 2)
 	buf := make([]byte, mtu+header.EthernetMinimumSize)
 
 	for {
-		n, err := io.ReadFull(conn, sizeBuf)
+		n, err := io.ReadFull(conn, size)
 		if err != nil {
 			errCh <- fmt.Errorf("failed to read frame size from connection: %w", err)
 			return
@@ -163,7 +155,7 @@ func tx(conn net.Conn, tap *water.Interface, errCh chan error, mtu int) {
 			errCh <- fmt.Errorf("received unexpected frame size %d", n)
 			return
 		}
-		size := int(binary.LittleEndian.Uint16(sizeBuf[0:2]))
+		size := int(binary.LittleEndian.Uint16(size[0:2]))
 
 		n, err = io.ReadFull(conn, buf[:size])
 		if err != nil {