nsfs: expose the stable inode numbers in a public header

Userspace heavily relies on the root inode numbers for namespaces 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 and align the only two namespaces that currently don't do that
with all the other namespaces.

To: linux-fsdevel@vger.kernel.org
To: netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

---
Changes in v2:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v1: https://lore.kernel.org/20250606-work-nsfs-v1-0-b8749c9a8844@kernel.org



--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
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    "change-id": "20250606-work-nsfs-3e951f888309",
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