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.
