unix: fix potential overflow in Mkdev on Linux

Daniel Swarbrick reports the following go vet warning after CL 50550:

  .../dev_linux.go:38: (major & 0xfffff000) (32 bits) too small for shift of 32

Fix it by converting major and minor to uint64 before the bitwise-and.

Change-Id: If37540be81f2c78a58fd5e5dcce7b770ce7b8afe
Reviewed-on: https://go-review.googlesource.com/63090
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/dev_linux.go

@@ -34,9 +34,9 @@ func Minor(dev uint64) uint32 {
 // Mkdev returns a Linux device number generated from the given major and minor
 // components.
 func Mkdev(major, minor uint32) uint64 {
-	dev := uint64((major & 0x00000fff) << 8)
+	dev := (uint64(major) & 0x00000fff) << 8
-	dev |= uint64((major & 0xfffff000) << 32)
+	dev |= (uint64(major) & 0xfffff000) << 32
-	dev |= uint64((minor & 0x000000ff) << 0)
+	dev |= (uint64(minor) & 0x000000ff) << 0
-	dev |= uint64((minor & 0xffffff00) << 12)
+	dev |= (uint64(minor) & 0xffffff00) << 12
 	return dev
 }