reiserfs: use bdev_nr_bytes instead of open coding it

reiserfs: use bdev_nr_bytes instead of open coding it

Use the proper helper to read the block device size and remove two
cargo culted checks that can't be false.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20211018101130.1838532-23-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>