partitions/ibm: use bdev_nr_sectors instead of open coding it

Use the proper helper to read the block device size and switch various
places to pass the size in terms of sectors which is more practical.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211019062024.2171074-4-hch@lst.de


[axboe: fix comment typo]
Signed-off-by: Jens Axboe <axboe@kernel.dk>