Merge patch series "nsfs: expose the stable inode numbers in a public header"

Christian Brauner <brauner@kernel.org> says:

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.

* patches from https://lore.kernel.org/20250606-work-nsfs-v1-0-b8749c9a8844@kernel.org:
  mntns: use stable inode number for initial mount ns
  netns: use stable inode number for initial mount ns
  nsfs: move root inode number to uapi

Link: https://lore.kernel.org/20250606-work-nsfs-v1-0-b8749c9a8844@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>