block/rnbd-clt-sysfs: use default_groups in kobj_type

There are currently 2 ways to create a set of sysfs files for a
kobj_type, through the default_attrs field, and the default_groups
field.  Move the rnbd controller sysfs code to use default_groups field
which has been the preferred way since aa30f47c

 ("kobject: Add
support for default attribute groups to kobj_type") so that we can soon
get rid of the obsolete default_attrs field.

Cc: "Md. Haris Iqbal" <haris.iqbal@ionos.com>
Cc: Jack Wang <jinpu.wang@ionos.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20220104162947.1320936-1-gregkh@linuxfoundation.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>