Comment # 26 on bug 1055641 from Franck Bui

(In reply to Franck Bui from comment #18)
> OK so apparently there's something wrong with /dev/dm-0.
> 

After some research it appears that the warnings:

  systemd-shutdown[1]: Detaching DM 254:0.
  systemd-shutdown[1]: Could not detach DM /dev/dm-0: Device or resource busy
  systemd-shutdown[1]: Not all DM devices detached, 1 left.

are emitted because we're trying to detach the device that contains the rootfs.
And this is doomed to fail as we're currently using it.

Normally we should detect this case and skip detaching the DM device hosting
the rootfs but in your case BTRFS is used and the detection was failing.

I submitted a patch to fix that:
https://github.com/systemd/systemd/pull/6811

However the original issue remains...