When cgroup_mount() fails to allocate an id for the root, it didn't
set ret before jumping to unlock_drop ending up returning 0 after a
failure.  Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: stable@vger.kernel.org