brd: get rid of unused members from struct brd_device

brd_refcnt, brd_offset, brd_sizelimit and brd_blocksize in struct
brd_device seem to be copied from struct loop_device but they're
not used anywhere. Let get rid of them.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>