block: replace bd_set_size with bd_set_nr_sectors

Replace bd_set_size with a version that takes the number of sectors
instead, as that fits most of the current and future callers much better.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>