selinux: allow changing labels for cgroupfs

This patch allows changing labels for cgroup mounts. Previously, running
chcon on cgroupfs would throw an "Operation not supported". This patch
specifically whitelist cgroupfs.

The patch could also allow containers to write only to the systemd cgroup
for instance, while the other cgroups are kept with cgroup_t label.

Signed-off-by: Antonio Murdaca <runcom@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>