cgfsng: only establish mapping once

cgfsng: only establish mapping once

When we deleted cgroups for unprivileged containers we used to allocate a new
mapping and clone a new user namespace each time we delete a cgroup. This of
course meant - on a cgroup v1 system - doing this >= 10 times when all
controllers were used. Let's not to do this and only allocate and establish a
mapping once.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>