ipv4: use unsafe.Sizeof on struct size, not pointer size

Fix for CL 203400.

Change-Id: Ia0006cdeff4992a218c7a355bd83354138706833
Reviewed-on: https://go-review.googlesource.com/c/net/+/203777
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ipv4/control_bsd.go

@@ -36,6 +36,6 @@ func marshalInterface(b []byte, cm *ControlMessage) []byte {
 
 func parseInterface(cm *ControlMessage, b []byte) {
 	var sadl syscall.SockaddrDatalink
-	copy((*[unsafe.Sizeof(&sadl)]byte)(unsafe.Pointer(&sadl))[:], b)
+	copy((*[unsafe.Sizeof(sadl)]byte)(unsafe.Pointer(&sadl))[:], b)
 	cm.IfIndex = int(sadl.Index)
 }