cgroups: improve container cgroup attaching

cgroups: improve container cgroup attaching

The current attach.c codepath which handles moving the attaching process into
the container's cgroups allocates a whole new struct cgroup_ops and goes
through the trouble of reparsing the whole cgroup layout.
That's costly and wasteful. My plan has always been to move this into the
command api by getting fds for attaching back but but it's not worth going
through that hazzle for non-unified hosts. On pure unified hosts however -
being the future - we can just attach through a single fd so there's no need to
allocate and setup struct cgroup_ops.

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