cgroup: prevent DOS when a hierachy is mounted multiple times

When starting a container, we walk through all cgroup mounts looking
for a unique directory name we can use for this container.  If the
name we are trying is in use, we try another name.  If it is not in
use in the first mount we check, we need to check other hierarchies
as it may exist there.  But we weren't checking whether we have already
checked a subsystem - so that if freezer was mounted twice, we would
create it in the first mount, see it exists in the second, so start
over trying in the second mount.

To fix this, keep track of which subsystems we have already checked,
and do not re-check.

(See http://pad.lv/1176287 for a bug report)

Note we still need to add, at the next: label, the removal of the
directories we've already created.  I'm keeping that for later as
it's far lower priority than this fix, and I don't want to risk
introducing a regression for that.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>