l2tp: remove incorrect __rcu attribute

This fixes a sparse warning.

Fixes: d18d3f0a ("l2tp: replace hlist with simple list for per-tunnel session list")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406220754.evK8Hrjw-lkp@intel.com/Signed-off-by: James Chapman <jchapman@katalix.com>
Link: https://patch.msgid.link/20240624082945.1925009-1-jchapman@katalix.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/l2tp/l2tp_core.c

@@ -1290,7 +1290,7 @@ static void l2tp_session_unhash(struct l2tp_session *session)
 static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel)
 {
 	struct l2tp_session *session;
-	struct list_head __rcu *pos;
+	struct list_head *pos;
 	struct list_head *tmp;
 
 	spin_lock_bh(&tunnel->list_lock);