ext4: use sb_bdev_nr_blocks

ext4: use sb_bdev_nr_blocks

Use the sb_bdev_nr_blocks helper instead of open coding it.

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>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20211018101130.1838532-27-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>