nsfs: add open_namespace()

and call it from open_related_ns().

Link: https://lore.kernel.org/r/20240627-work-pidfs-v1-3-7e9ab6cc3bb1@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>