block: merge disk_scan_partitions and blkdev_reread_part

Unify the functionality that implements a partition rescan for a
gendisk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211122130625.1136848-6-hch@lst.de


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