ipv6: release nexthop on device removal

The CI is hitting some aperiodic hangup at device removal time in the
pmtu.sh self-test:

unregister_netdevice: waiting for veth_A-R1 to become free. Usage count = 6
ref_tracker: veth_A-R1@ffff888013df15d8 has 1/5 users at
	dst_init+0x84/0x4a0
	dst_alloc+0x97/0x150
	ip6_dst_alloc+0x23/0x90
	ip6_rt_pcpu_alloc+0x1e6/0x520
	ip6_pol_route+0x56f/0x840
	fib6_rule_lookup+0x334/0x630
	ip6_route_output_flags+0x259/0x480
	ip6_dst_lookup_tail.constprop.0+0x5c2/0x940
	ip6_dst_lookup_flow+0x88/0x190
	udp_tunnel6_dst_lookup+0x2a7/0x4c0
	vxlan_xmit_one+0xbde/0x4a50 [vxlan]
	vxlan_xmit+0x9ad/0xf20 [vxlan]
	dev_hard_start_xmit+0x10e/0x360
	__dev_queue_xmit+0xf95/0x18c0
	arp_solicit+0x4a2/0xe00
	neigh_probe+0xaa/0xf0

While the first suspect is the dst_cache, explicitly tracking the dst
owing the last device reference via probes proved such dst is held by
the nexthop in the originating fib6_info.

Similar to commit f5b51fe ("ipv6: route: purge exception on
removal"), we need to explicitly release the originating fib info when
disconnecting a to-be-removed device from a live ipv6 dst: move the
fib6_info cleanup into ip6_dst_ifdown().

Tested running:

./pmtu.sh cleanup_ipv6_exception

in a tight loop for more than 400 iterations with no spat, running an
unpatched kernel  I observed a splat every ~10 iterations.

Fixes: f88d8ea ("ipv6: Plumb support for nexthop object in a fib6_info")
Signed-off-by: Paolo Abeni <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Link: https://patch.msgid.link/604c45c188c609b732286b47ac2a451a40f6cf6d.1730828007.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Paolo Abeni

net/ipv6/route.c

@@ -374,6 +374,7 @@ static void ip6_dst_ifdown(struct dst_entry *dst, struct net_device *dev)
 {
 	struct rt6_info *rt = dst_rt6_info(dst);
 	struct inet6_dev *idev = rt->rt6i_idev;
+	struct fib6_info *from;
 
 	if (idev && idev->dev != blackhole_netdev) {
 		struct inet6_dev *blackhole_idev = in6_dev_get(blackhole_netdev);
@@ -383,6 +384,8 @@ static void ip6_dst_ifdown(struct dst_entry *dst, struct net_device *dev)
 			in6_dev_put(idev);
 		}
 	}
+	from = unrcu_pointer(xchg(&rt->from, NULL));
+	fib6_info_release(from);
 }
 
 static bool __rt6_check_expired(const struct rt6_info *rt)
@@ -1455,7 +1458,6 @@ static DEFINE_SPINLOCK(rt6_exception_lock);
 static void rt6_remove_exception(struct rt6_exception_bucket *bucket,
 				 struct rt6_exception *rt6_ex)
 {
-	struct fib6_info *from;
 	struct net *net;
 
 	if (!bucket || !rt6_ex)
@@ -1467,8 +1469,6 @@ static void rt6_remove_exception(struct rt6_exception_bucket *bucket,
 	/* purge completely the exception to allow releasing the held resources:
 	 * some [sk] cache may keep the dst around for unlimited time
 	 */
-	from = unrcu_pointer(xchg(&rt6_ex->rt6i->from, NULL));
-	fib6_info_release(from);
 	dst_dev_put(&rt6_ex->rt6i->dst);
 
 	hlist_del_rcu(&rt6_ex->hlist);