bdev: make bdev_{release, open_by_dev}() private to block layer

Move both of them to the private block header. There's no caller in the
tree anymore that uses them directly.

Link: https://lore.kernel.org/r/20240123-vfs-bdev-file-v2-28-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>