zram: Convert to use bdev_open_by_dev()

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

CC: Minchan Kim <minchan@kernel.org>
CC: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230927093442.25915-8-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>