net: fix WriteTo on Plan 9

This change excludes internal UDP header size from a result of number of
bytes written on WriteTo.

Change-Id: I847d57f7f195657b6f14efdf1b4cfab13d4490dd
Reviewed-on: https://go-review.googlesource.com/9196Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/net/udpsock_plan9.go

@@ -86,11 +86,10 @@ func (c *UDPConn) WriteToUDP(b []byte, addr *UDPAddr) (int, error) {
 	buf := make([]byte, udpHeaderSize+len(b))
 	i := copy(buf, h.Bytes())
 	copy(buf[i:], b)
-	n, err := c.fd.data.Write(buf)
-	if err != nil {
-		err = &OpError{Op: "write", Net: c.fd.dir, Addr: addr, Err: err}
+	if _, err := c.fd.data.Write(buf); err != nil {
+		return 0, &OpError{Op: "write", Net: c.fd.dir, Addr: addr, Err: err}
 	}
-	return n, err
+	return len(b), nil
 }
 
 // WriteTo implements the PacketConn WriteTo method.