block: rbd: convert to use BUS_ATTR_WO and RO

We are trying to get rid of BUS_ATTR() and the usage of that in rbd.c
can be trivially converted to use BUS_ATTR_WO and RO, so use those
macros instead.

Cc: Sage Weil <sage@redhat.com>
Cc: Alex Elder <elder@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Acked-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>