syscall: fix vet warning in UnixRights

Change-Id: Ic19c94fe0af55e17f6c2fcfd36085ccb1584da6f
Reviewed-on: https://go-review.googlesource.com/15608Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/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
 }