block: fix bdev_disk_changed for non-partitioned devices
We still have to set the capacity to 0 if invalidating or call
revalidate_disk if not even if the disk has no partitions. Fix
that by merging rescan_partitions into bdev_disk_changed and just
stubbing out blk_add_partitions and blk_drop_partitions for
non-partitioned devices.
Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Jens Axboe <axboe@kernel.dk>