partitions/efi: use bdev_nr_bytes instead of open coding it

Use the proper helper to read the block device size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211019062024.2171074-3-hch@lst.de


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