pam: fix race in cgroup creation

pam: fix race in cgroup creation

If we find that a cgroup already exists, we should

1. remove all the cgroups which we've created so far
2. set existed to true
3. return failure

The caller should then detect that existed == true,
and re-try with the next index.

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