Device mapper was creating an invalid cgroup name.

This was breaking docker top CONTAINER

Docker-DCO-1.1-Signed-off-by: Daniel Walsh <dwalsh@redhat.com> (github: rhatdan)

We have had discussions on this, and some people have suggested we need a better long term solution, but this is broken in current docker when running on a systemd based system and executing docker top.

I think we should merge this fix until someone builds a long term solution