Before it was placed in /dev/disk/by-id, which makes it a bit too much
API. However, it's mostly an implementation detail for now, hence move
it out of the stable block device dir.
udev seems to have a race condition with swapon to see which can open
/dev/zram0 first, causing swapon to fail. Seems to be most noticeable
on arm devices one out of every 7 times or something.