xfrm: fix missing dst_release() after policy blocking lbcast and multicast

Fix missing dst_release() when local broadcast or multicast traffic is
xfrm policy blocked.

For IPv4 this results to dst leak: ip_route_output_flow() allocates
dst_entry via __ip_route_output_key() and passes it to
xfrm_lookup_route(). xfrm_lookup returns ERR_PTR(-EPERM) that is
propagated. The dst that was allocated is never released.

IPv4 local broadcast testcase:
 ping -b 192.168.1.255 &
 sleep 1
 ip xfrm policy add src 0.0.0.0/0 dst 192.168.1.255/32 dir out action block

IPv4 multicast testcase:
 ping 224.0.0.1 &
 sleep 1
 ip xfrm policy add src 0.0.0.0/0 dst 224.0.0.1/32 dir out action block

For IPv6 the missing dst_release() causes trouble e.g. when used in netns:
 ip netns add TEST
 ip netns exec TEST ip link set lo up
 ip link add dummy0 type dummy
 ip link set dev dummy0 netns TEST
 ip netns exec TEST ip addr add fd00::1111 dev dummy0
 ip netns exec TEST ip link set dummy0 up
 ip netns exec TEST ping -6 -c 5 ff02::1%dummy0 &
 sleep 1
 ip netns exec TEST ip xfrm policy add src ::/0 dst ff02::1 dir out action block
 wait
 ip netns del TEST

After netns deletion we see:
[  258.239097] unregister_netdevice: waiting for lo to become free. Usage count = 2
[  268.279061] unregister_netdevice: waiting for lo to become free. Usage count = 2
[  278.367018] unregister_netdevice: waiting for lo to become free. Usage count = 2
[  288.375259] unregister_netdevice: waiting for lo to become free. Usage count = 2

Fixes: ac37e251 ("xfrm: release dst_orig in case of error in xfrm_lookup()")
Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -2286,6 +2286,9 @@ struct dst_entry *xfrm_lookup_route(struct net *net, struct dst_entry *dst_orig,
 	if (IS_ERR(dst) && PTR_ERR(dst) == -EREMOTE)
 		return make_blackhole(net, dst_orig->ops->family, dst_orig);
 
+	if (IS_ERR(dst))
+		dst_release(dst_orig);
+
 	return dst;
 }
 EXPORT_SYMBOL(xfrm_lookup_route);