ipv4: Call rt_add_uncached_list() if !do_cache in ip_route_input_slow().

Since the cited commit, ip_route_input_slow() could call
rt_dst_alloc() with vrf in the local_input: path.

If res->fi != NULL and one of the following is true,

  - a non-zero itag from fib_validate_source_reason()
    (realms/classid)
  - RTN_BROADCAST with BC_FORWARDING enabled

do_cache is set to false.

Then, the route is not added to rt_uncached_list, so the
route cannot be removed from rt_flush_dev().

If the route is cached in sk->sk_rx_dst, removing the
vrf device gets stuck. [0]

Let's call rt_add_uncached_list() if !do_cache in
ip_route_input_slow().

[0]:
unregister_netdevice: waiting for vrf0 to become free. Usage count = 2
ref_tracker: netdev@000000008b304ac4 has 1/1 users at
     dst_alloc+0x14b/0x490
     ip_route_input_rcu+0x1540/0x2530
     ip_route_input_noref+0x5a/0x90
     ip_rcv_finish_core+0x4b0/0x1140
     ip_rcv_finish+0xe6/0x190
     ip_rcv+0x6c/0x130
     __netif_receive_skb+0x121/0x300
     process_backlog+0x2df/0x5e0
     __napi_poll+0x8c/0x380
     net_rx_action+0x43c/0xab0
     handle_softirqs+0x19f/0x520
     do_softirq+0x46/0x60
     __local_bh_enable_ip+0x79/0x80
     __dev_queue_xmit+0x1b0e/0x24c0
     ip_finish_output2+0x8b2/0xcd0
     ip_output+0xf2/0x1e0

Fixes: f5a0aab84b ("net: ipv4: dst for local input routes should use l3mdev if relevant")
Reported-by: Jason Zhang <jdz@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20261101064418.2055158-4-kuniyu@google.com
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv4/route.c

@@ -2494,6 +2494,8 @@ ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 
 		if (unlikely(!rt_cache_route(nhc, rth)))
 			rt_add_uncached_list(rth);
+	} else {
+		rt_add_uncached_list(rth);
 	}
 	skb_dst_set(skb, &rth->dst);
 	reason = SKB_NOT_DROPPED_YET;