cgroup: fix incorrect WARN_ON_ONCE() in cgroup_setup_root()

74321038 ("cgroup: use cgrp->kn->id as the cgroup ID") added WARN
which triggers if cgroup_id(root_cgrp) is not 1.  This is fine on
64bit ino archs but on 32bit archs cgroup ID is ((gen << 32) | ino)
and gen starts at 1, so the root id is 0x1_0000_0001 instead of 1
always triggering the WARN.

What we wanna make sure is that the ino part is 1.  Fix it.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: 74321038 ("cgroup: use cgrp->kn->id as the cgroup ID")
Signed-off-by: Tejun Heo <tj@kernel.org>