rust: configfs: fix data offset calculation for subsystem callbacks

`get_group_data()` chooses between `Group::<Parent>::container_of()` and
`Subsystem::<Parent>::container_of()` based on whether `this` represents
the root group of a configfs subsystem. It detects this by checking
`(*this).cg_subsys.is_null()`, but `link_group()` in `fs/configfs/dir.c`
unconditionally sets `cg_subsys` for every `config_group` attached anywhere
in a registered subsystem, including the subsystem's own `su_group`. The
only `config_group` with a NULL `cg_subsys` is the configfs root, on which
userspace cannot trigger callbacks. The check is therefore always false at
runtime, and the `Subsystem` branch is dead. Subsystem-level callbacks
reach the `Group` branch and may read `data` at the wrong offset.

Thus change the `is_root` check to correctly identify whether a group is a
root group by comparing `this` to `subsys.su_group`.

Cc: stable@vger.kernel.org
Fixes: 446cafc295bf ("rust: configfs: introduce rust support for configfs")
Link: https://msgid.link/20260608-configfs-fix-offset-v1-1-7f01b8fb9e5f@kernel.org
[ Andreas: Fix clippy warning on 1.85.0 by moving safety comment. ]
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

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