Invert they way the holder relations are tracked.  This very
slightly reduces the memory overhead for partitioned devices.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210804094147.459763-4-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>