cgroup is labeled from genfs_contexts. Also, cgroup filesystems can't be
context mounted, i.e. it's not possible to mount them with a label other
than "cgroup".

Bug: 110962171
Test: m selinux_policy
Test: boot aosp_walleye
Change-Id: I8319b10136c42a42d1edaee47b77ad1698e87f2c