Use the generic setup_bdev_super helper to open the main block device
and do various bits of superblock setup instead of duplicating the
logic.  This includes moving to the new scheme implemented in common
code that only opens the block device after the superblock has allocated.

It does not yet convert nilfs2 to the new mount API, but doing so will
become a bit simpler after this first step.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Message-Id: <20230802154131.2221419-3-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>