CVE-2021-47579 ovl: fix warning in ovl_create_real()

In the Linux kernel, the following vulnerability has been resolved:

ovl: fix warning in ovl_create_real()

Syzbot triggered the following warning in ovl_workdir_create() ->
ovl_create_real():

if (!err && WARN_ON(!newdentry->d_inode)) {

The reason is that the cgroup2 filesystem returns from mkdir without
instantiating the new dentry.

Weird filesystems such as this will be rejected by overlayfs at a later
stage during setup, but to prevent such a warning, call ovl_mkdir_real()
directly from ovl_workdir_create() and reject this case early.