Merge patch series "ns: rework common initialization"

Christian Brauner <brauner@kernel.org> says:

The current scheme still involves a lot of open-coding and copy-pasing
and bleeds a lot of unnecessary details into actual namespace
implementers. Encapsulate it in the common helpers and simplify it all.

* patches from https://lore.kernel.org/20250917-work-namespace-ns_common-v1-0-1b3bda8ef8f2@kernel.org:
  ns: add ns_common_free()
  nscommon: simplify initialization
  net: centralize ns_common initialization
  mnt: simplify ns_common_init() handling
  nsfs: add inode number for anon namespace
  cgroup: split namespace into separate header
  nscommon: move to separate file
  mnt: expose pointer to init_mnt_ns
  uts: split namespace into separate header

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