block: rbd: make rbd_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the rbd_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20240204-bus_cleanup-block-v1-1-fc77afd8d7cc@marliere.net


Signed-off-by: Jens Axboe <axboe@kernel.dk>