Convert ext4 to use bdev_open_by_dev() and pass the handle around.

CC: linux-ext4@vger.kernel.org
CC: Ted Tso <tytso@mit.edu>
Acked-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230927093442.25915-22-jack@suse.czSigned-off-by: Christian Brauner <brauner@kernel.org>