mtd: block2mtd: Convert to bdev_open_by_dev/path()

Convert block2mtd to use bdev_open_by_dev() and bdev_open_by_path() and
pass the handle around.

CC: Joern Engel <joern@lazybastard.org>
CC: linux-mtd@lists.infradead.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/20230927093442.25915-12-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>