unix: avoid "just past the end" pointers in UnixRights

Same as CL 201617 did for package syscall.

Caught with -d=checkptr

Updates golang/go#22218

Change-Id: I8208f8e6d9bd62376bf9e0458dc18956daabd785
Reviewed-on: https://go-review.googlesource.com/c/sys/+/201937
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <mail@eliasnaur.com>

unix/sockcmsg_linux.go

@@ -17,7 +17,7 @@ func UnixCredentials(ucred *Ucred) []byte {
 	h.Level = SOL_SOCKET
 	h.Type = SCM_CREDENTIALS
 	h.SetLen(CmsgLen(SizeofUcred))
-	*((*Ucred)(cmsgData(h))) = *ucred
+	*(*Ucred)(h.data(0)) = *ucred
 	return b
 }