sysfs: attribute_group: allow registration of const attribute

To be able to constify instances of struct attribute it has to be
possible to add them to struct attribute_group. The current type of the
attrs member however is not compatible with that. Introduce a union that
allows registration of both const and non-const attributes to enable a
piecewise transition. As both union member types are compatible no
logic needs to be adapted.

Technically it is now possible register a const struct attribute and
receive it as mutable pointer in the callbacks. This is a soundness
issue. But this same soundness issue already exists today in
sysfs_create_file(). Also the struct definition and callback
implementation are always closely linked and are meant to be moved to
const in lockstep.

Similar to commit 906c508afdca ("sysfs: attribute_group: allow
registration of const bin_attribute")

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20251029-sysfs-const-attr-prep-v5-1-ea7d745acff4@weissschuh.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>