cgroup: collapse cgroup_create() into croup_mkdir()

cgroup_mkdir() is the sole user of cgroup_create().  Let's collapse
the latter into the former.  This will help simplifying locking.
While at it, remove now stale comment about inode locking.

This patch doesn't introduce any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>