[release-branch.go1.4] syscall: fix the deprecated way of parsing routing message on openbsd

OpenBSD 5.5 changed its kernel ABI and OpenBSD 5.6 enabled it.
This CL works on both 5.5 and 5.6.

Fixes #9102.

Change-Id: I4a295be9ab8acbc99e550d8cb7e8f8dacf3a03c5
Reviewed-on: https://go-review.googlesource.com/1932Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 13e16b39)
Reviewed-on: https://go-review.googlesource.com/2826Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/route_openbsd.go

@@ -12,16 +12,16 @@ func (any *anyMessage) toRoutingMessage(b []byte) RoutingMessage {
 	switch any.Type {
 	case RTM_ADD, RTM_DELETE, RTM_CHANGE, RTM_GET, RTM_LOSING, RTM_REDIRECT, RTM_MISS, RTM_LOCK, RTM_RESOLVE:
 		p := (*RouteMessage)(unsafe.Pointer(any))
-		return &RouteMessage{Header: p.Header, Data: b[SizeofRtMsghdr:any.Msglen]}
+		return &RouteMessage{Header: p.Header, Data: b[p.Header.Hdrlen:any.Msglen]}
 	case RTM_IFINFO:
 		p := (*InterfaceMessage)(unsafe.Pointer(any))
-		return &InterfaceMessage{Header: p.Header, Data: b[SizeofIfMsghdr:any.Msglen]}
+		return &InterfaceMessage{Header: p.Header, Data: b[p.Header.Hdrlen:any.Msglen]}
 	case RTM_IFANNOUNCE:
 		p := (*InterfaceAnnounceMessage)(unsafe.Pointer(any))
 		return &InterfaceAnnounceMessage{Header: p.Header}
 	case RTM_NEWADDR, RTM_DELADDR:
 		p := (*InterfaceAddrMessage)(unsafe.Pointer(any))
-		return &InterfaceAddrMessage{Header: p.Header, Data: b[SizeofIfaMsghdr:any.Msglen]}
+		return &InterfaceAddrMessage{Header: p.Header, Data: b[p.Header.Hdrlen:any.Msglen]}
 	}
 	return nil
 }