md: use bdev_nr_sectors instead of open coding it

Use the proper helper to read the block device size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20211018101130.1838532-7-hch@lst.de


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