dm: delay registering the gendisk

device mapper is currently the only outlier that tries to call
register_disk after add_disk, leading to fairly inconsistent state
of these block layer data structures.  Instead change device-mapper
to just register the gendisk later now that the holder mechanism
can cope with that.

Note that this introduces a user visible change: the dm kobject is
now only visible after the initial table has been loaded.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20210804094147.459763-8-hch@lst.de


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