block: add a bdev_nr_bytes helper

Add a helper to query the size of a block device in bytes.  This
will be used to remove open coded access to ->bd_inode.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211018101130.1838532-3-hch@lst.de


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