ipv6: fix possible infinite loop in fib6_info_uses_dev()

fib6_info_uses_dev() seems to rely on RCU without an explicit
protection.

Like the prior fix in rt6_nlmsg_size(),
we need to make sure fib6_del_route() or fib6_add_rt2node()
have not removed the anchor from the list, or we risk an infinite loop.

Fixes: d9ccb18 ("ipv6: Fix soft lockups in fib6_select_path under high next hop churn")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250725140725.3626540-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv6/route.c

@@ -5958,16 +5958,21 @@ static bool fib6_info_uses_dev(const struct fib6_info *f6i,
 	if (f6i->fib6_nh->fib_nh_dev == dev)
 		return true;
 
-	if (f6i->fib6_nsiblings) {
-		struct fib6_info *sibling, *next_sibling;
+	if (READ_ONCE(f6i->fib6_nsiblings)) {
+		const struct fib6_info *sibling;
 
-		list_for_each_entry_safe(sibling, next_sibling,
-					 &f6i->fib6_siblings, fib6_siblings) {
-			if (sibling->fib6_nh->fib_nh_dev == dev)
+		rcu_read_lock();
+		list_for_each_entry_rcu(sibling, &f6i->fib6_siblings,
+					fib6_siblings) {
+			if (sibling->fib6_nh->fib_nh_dev == dev) {
+				rcu_read_unlock();
 				return true;
+			}
+			if (!READ_ONCE(f6i->fib6_nsiblings))
+				break;
 		}
+		rcu_read_unlock();
 	}
-
 	return false;
 }