ns: initialize ns_list_node for initial namespaces

ns: initialize ns_list_node for initial namespaces

[ Upstream commit 3dd50c58664e2684bd610a57bf3ab713cbb0ea91 ]

Make sure that the list is always initialized for initial namespaces.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-8-2e6f823ebdc0@kernel.org
Fixes: 885fc8ac0a4d ("nstree: make iterator generic")
Tested-by: syzbot@syzkaller.appspotmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>