nsfs: move root inode number to uapi

Userspace relies on the root inode numbers to identify the initial
namespaces. That's already a hard dependency. So we cannot change that
anymore. Move the initial inode numbers to a public header.

Link: https://github.com/systemd/systemd/commit/d293fade24b34ccc2f5716b0ff5513e9533cf0c4
Link: https://lore.kernel.org/20250606-work-nsfs-v1-1-b8749c9a8844@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>