pcd: cleanup initialization

[ Upstream commit af761f27 ]

Refactor the pcd initialization to have a dedicated helper to initialize
a single disk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: 1a721de8 ("block: don't add or resize partition on the disk with GENHD_FL_NO_PART")
Signed-off-by: Sasha Levin <sashal@kernel.org>