floppy: use block_size

Use the block_size helper instead of open coding it.  Also remove the
check for a 0 block size, as that can't happen.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>