ipv6: add exception routes to GC list in rt6_insert_exception

Commit 5eb902b ("net/ipv6: Remove expired routes with a separated list
of routes.") introduced a separated list for managing route expiration via
the GC timer.

However, it missed adding exception routes (created by ip6_rt_update_pmtu()
and rt6_do_redirect()) to this GC list. As a result, these exceptions were
never considered for expiration and removal, leading to stale entries
persisting in the routing table.

This patch fixes the issue by calling fib6_add_gc_list() in
rt6_insert_exception(), ensuring that exception routes are properly tracked
and garbage collected when expired.

Fixes: 5eb902b ("net/ipv6: Remove expired routes with a separated list of routes.")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/837e7506ffb63f47faa2b05d9b85481aad28e1a4.1744134377.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/route.c

@@ -1771,6 +1771,7 @@ static int rt6_insert_exception(struct rt6_info *nrt,
 	if (!err) {
 		spin_lock_bh(&f6i->fib6_table->tb6_lock);
 		fib6_update_sernum(net, f6i);
+		fib6_add_gc_list(f6i);
 		spin_unlock_bh(&f6i->fib6_table->tb6_lock);
 		fib6_force_start_gc(net);
 	}