bcache: Convert to bdev_open_by_path()

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

CC: linux-bcache@vger.kernel.org
CC: Coly Li <colyli@suse.de>
CC: Kent Overstreet <kent.overstreet@gmail.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230927093442.25915-9-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>