md: Convert to bdev_open_by_dev()

Convert md to use bdev_open_by_dev() and pass the handle around. We also
don't need the 'Holder' flag anymore so remove it.

CC: linux-raid@vger.kernel.org
CC: Song Liu <song@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230927093442.25915-11-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>