Large parts of rescan_partitions aren't about partitions, and
moving it to block_dev.c will allow for some further cleanups by
merging it into its only caller.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NJens Axboe <axboe@kernel.dk>