mkdir: don't do it as setresid

mkdir: don't do it as setresid

First of all the code was wrong anyway.

Secondly, doing mkdir under setres{ug}id won't work because that
won't respect privilege that callers in user namespaces may have.

We check the permissions meticulously anyway, so do the mkdir as
root and then chown the new cgroup.

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