syscall: fix uint64->int cast of control message header

Change-Id: I28980b307d10730b122a4f833809bc400d6aff24
Reviewed-on: https://go-review.googlesource.com/22525Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/sockcmsg_unix.go

@@ -62,7 +62,7 @@ func ParseSocketControlMessage(b []byte) ([]SocketControlMessage, error) {
 
 func socketControlMessageHeaderAndData(b []byte) (*Cmsghdr, []byte, error) {
 	h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
-	if h.Len < SizeofCmsghdr || int(h.Len) > len(b) {
+	if h.Len < SizeofCmsghdr || uint64(h.Len) > uint64(len(b)) {
 		return nil, nil, EINVAL
 	}
 	return h, b[cmsgAlignOf(SizeofCmsghdr):h.Len], nil