The current implementation assure that the major and the minor are
coded on one byte. But they are not:

```
brw-rw----  1 root disk    252, 290 Feb 25 11:36 dm-290
```

290 as minor in this example is over 1 byte.

So after wondering why all my devices iops weren't correctly stored,
I found out that several points were added for some disks. For `dm-290`
it was overriding `252:34`, instead of getting udev stats for `252:290`.

The solution is here:
https://sites.uclouvain.be/SystInfo/usr/include/sys/sysmacros.h.html

The implementation is directly taken from this, fixing my bug.

For most platforms, stat.Rdev is already a uint64 so this is without any effect
for linux,mipsle, unix.Stat_t.Rdev is a uint32, but the way to compute major and minor doesn't change, casting the uint32 has no impact either