unix: convert major/minor to uint64 before shifting in Mkdev on Darwin/*BSD

Follow CL 63090 and change the Makedev function on Darwin and *BSD to
convert to uint64 before shifting/masking. This avoids a potential
overflow.

Change-Id: I5b566329695cc5edcf82f0ff2366033011b0625b
Reviewed-on: https://go-review.googlesource.com/63112Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/dev_darwin.go

@@ -20,5 +20,5 @@ func Minor(dev uint64) uint32 {
 // Mkdev returns a Darwin device number generated from the given major and minor
 // components.
 func Mkdev(major, minor uint32) uint64 {
-	return uint64((major << 24) | minor)
+	return (uint64(major) << 24) | uint64(minor)
 }

unix/dev_dragonfly.go

@@ -26,5 +26,5 @@ func Minor(dev uint64) uint32 {
 // Mkdev returns a DragonFlyBSD device number generated from the given major and
 // minor components.
 func Mkdev(major, minor uint32) uint64 {
-	return uint64((major << 8) | minor)
+	return (uint64(major) << 8) | uint64(minor)
 }

unix/dev_freebsd.go

@@ -26,5 +26,5 @@ func Minor(dev uint64) uint32 {
 // Mkdev returns a FreeBSD device number generated from the given major and
 // minor components.
 func Mkdev(major, minor uint32) uint64 {
-	return uint64((major << 8) | minor)
+	return (uint64(major) << 8) | uint64(minor)
 }

unix/dev_netbsd.go

@@ -22,8 +22,8 @@ func Minor(dev uint64) uint32 {
 // Mkdev returns a NetBSD device number generated from the given major and minor
 // components.
 func Mkdev(major, minor uint32) uint64 {
-	dev := uint64((major << 8) & 0x000fff00)
-	dev |= uint64((minor << 12) & 0xfff00000)
-	dev |= uint64((minor << 0) & 0x000000ff)
+	dev := (uint64(major) << 8) & 0x000fff00
+	dev |= (uint64(minor) << 12) & 0xfff00000
+	dev |= (uint64(minor) << 0) & 0x000000ff
 	return dev
 }