fs: add vfs_cmd_create()

Split the steps to create a superblock into a tiny helper. This will
make the next patch easier to follow.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Aleksa Sarai <cyphar@cyphar.com>
Message-Id: <20230802-vfs-super-exclusive-v2-2-95dc4e41b870@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>