use RawConn.Control to get fd instead of Fd()

[LinkShen]

file.Fd() has a potential bug when work with unblocking connection which is the default setting of Golang lib.

// Fd returns the integer Unix file descriptor referencing the open file.
// The file descriptor is valid only until f.Close is called or f is garbage collected.
// On Unix systems this will cause the SetDeadline methods to stop working.
func (f *File) Fd() uintptr {
	if f == nil {
		return ^(uintptr(0))
	}

	// If we put the file descriptor into nonblocking mode,
	// then set it to blocking mode before we return it,
	// because historically we have always returned a descriptor
	// opened in blocking mode. The File will continue to work,
	// but any blocking operation will tie up a thread.
	if f.nonblock {
		f.pfd.SetBlocking() // HERE!!!
	}

	return uintptr(f.pfd.Sysfd)
}

This will case tcpconn becoming blocking, and may case goroutine blocked at conn.Close().
check this link for more information: golang/go#29277

So i use RawConn.Control which used in many Golang lib to do the syscall.

[brucespang]

Looks good to me, thank you for the patch!