md: port block device access to file

Link: https://lore.kernel.org/r/20240123-vfs-bdev-file-v2-4-adbd023e19cc@kernel.org


Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>