brd: add ram disk visibility option

Currenly ram disk is not visiable inside /proc/partitions. This was
done for compatibility reasons here: 53978d0a. But some utilities
expect disk presents in /proc/partitions.
Let's add module's option and let's administrator chose visibility behaviour.
By default, old behaviour preserved.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jens Axboe <axboe@fb.com>