sysfs: attribute_group: allow registration of const bin_attribute

sysfs: attribute_group: allow registration of const bin_attribute

To be able to constify instances of struct bin_attribute it has to be
possible to add them to string attribute_group.
The current type of the bin_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
bin_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_bin_file().
Also the struct definition and callback implementation are always
closely linked and are meant to be moved to const in lockstep.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20241115-b4-sysfs-const-bin_attr-group-v1-1-2c9bb12dfc48@weissschuh.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>