cgfsng: ensure no-reuse in cgfsng_monitor_create()

cgfsng: ensure no-reuse in cgfsng_monitor_create()

The same way we need to ensure that no existing cgroups are reused for
the payload in cgfsng_payload_create() we need to ensure that no
existing cgroups are reused for the monitor. Technially this is less of
an issue since there currently is no logic for the monitor to apply
limits to its cgroup but it is still the proper way to do it.

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