block: add a loff_t cast to bdev_nr_bytes

Not really needed as both loff_t and sector_t are always 64-bits wide,
but this documents the different types a bit better.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211104172037.531803-1-hch@lst.de


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