block: fold register_disk into device_add_disk

mainline inclusion
from mainline-v5.15-rc1
commit 52b85909
category: bugfix
bugzilla: 188733

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=52b85909f85d06efa69aaf4210e72467f1f58d2b



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There is no real reason these should be separate.  Also simplify the
groups assignment a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210818144542.19305-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>
conflict:
  block/genhd.c
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Signed-off-by: Li Nan <linan122@huawei.com>