Merge patch series "ns: rework reference counting"

Christian Brauner <brauner@kernel.org> says:

Stop open accesses to the reference counts and cargo-culting the same
code in all namespace. Use a set of dedicated helpers and make the
actual count private.

* patches from https://lore.kernel.org/20250918-work-namespace-ns_ref-v1-0-1b0a98ee041e@kernel.org:
  ns: rename to __ns_ref
  nsfs: port to ns_ref_*() helpers
  net: port to ns_ref_*() helpers
  uts: port to ns_ref_*() helpers
  ipv4: use check_net()
  net: use check_net()
  net-sysfs: use check_net()
  user: port to ns_ref_*() helpers
  time: port to ns_ref_*() helpers
  pid: port to ns_ref_*() helpers
  ipc: port to ns_ref_*() helpers
  cgroup: port to ns_ref_*() helpers
  mnt: port to ns_ref_*() helpers
  ns: add reference count helpers

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