unix: fix vet warning in UnixRights

Same fix as golang.org/cl/15608 for package syscall.

Change-Id: I8de2369220e58140596325d7ea16c015aab90d1b
Reviewed-on: https://go-review.googlesource.com/15609
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/sockcmsg_unix.go

@@ -77,10 +77,10 @@ func UnixRights(fds ...int) []byte {
 	h.Level = SOL_SOCKET
 	h.Type = SCM_RIGHTS
 	h.SetLen(CmsgLen(datalen))
-	data := uintptr(cmsgData(h))
+	data := cmsgData(h)
 	for _, fd := range fds {
-		*(*int32)(unsafe.Pointer(data)) = int32(fd)
-		data += 4
+		*(*int32)(data) = int32(fd)
+		data = unsafe.Pointer(uintptr(data) + 4)
 	}
 	return b
 }