block: remove (__)blkdev_reread_part as an exported API

In general drivers should never mess with partition tables directly.
Unfortunately s390 and loop do for somewhat historic reasons, but they
can use bdev_disk_changed directly instead when we export it as they
satisfy the sanity checks we have in __blkdev_reread_part.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>	[dasd]
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>