Merge patch series "ns: tweak ns common handling"

Christian Brauner <brauner@kernel.org> says:

This contains three minor tweaks for namespace handling:

* Make struct ns_tree private. There's no need for anything to access
  that directly.

* Drop a debug assert that would trigger in conditions that are benign.

* Move the type of the namespace out of struct proc_ns_operations and
  into struct ns_common. This eliminates a pointer dereference and also
  allows assertions to work when the namespace type is disabled and the
  operations field set to NULL.

* patches from https://lore.kernel.org/20250924-work-namespaces-fixes-v1-0-8fb682c8678e@kernel.org:
  ns: drop assert
  ns: move ns type into struct ns_common
  nstree: make struct ns_tree private

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