block: add a sb_bdev_nr_blocks helper

Add a helper to return the size of sb->s_bdev in sb->s_blocksize_bits
based unites.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20211018101130.1838532-26-hch@lst.de


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