ns: use anonymous struct to group list member
Make it easier to spot that they belong together conceptually.
Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-12-2e6f823ebdc0@kernel.org
Tested-by: syzbot@syzkaller.appspotmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>