selinux: allow per-file labeling for cgroupfs

This patch allows genfscon per-file labeling for cgroupfs. For instance,
this allows to label the "release_agent" file within each
cgroup mount and limit writes to it.

Signed-off-by: Antonio Murdaca <amurdaca@redhat.com>
[PM: subject line and merge tweaks]
Signed-off-by: Paul Moore <paul@paul-moore.com>