[OVN] Unable to ping router from test IPV6 subnet

Steps to reproduce:

1) Create test network, IPV6 subnet in arbitrary mode, eg. SLAAC;
2) Create test router and add an interface in the test subnet;
3) Create test instance and try to ping the router - it fails.

Sample OVN trace:

# ovn-trace piotr-geneve-ipv6-stateful 'inport == "9b6d8c19-ca57-4f0d-8af0-ee29642c6fb8" && eth.src == fa:16:3e:f5:aa:2a && eth.dst == fa:16:3e:6b:1b:79 && ip6.src == 2a01:49a0:112a::289 && ip6.dst == 2a01:49a0:112a::'

 0. ls_out_pre_lb (ovn-northd.c:5169): ip && outport == "7f82a5", priority 110, uuid 4cb710e8
    next;
 1. ls_out_pre_acl (ovn-northd.c:5169): ip && outport == "7f82a5", priority 110, uuid 65a121ee
    next;
 4. ls_out_acl_hint (ovn-northd.c:5486): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0, priority 4, uuid 99195420
    reg0[8] = 1;
    reg0[10] = 1;
    next;
10. ls_out_port_sec_l2 (ovn-northd.c:5118): outport == "7f82a5", priority 50, uuid cc53fba5
    output;
    /* output to "7f82a5", type "patch" */

ingress(dp="test-router", inport="lrp-7f82a5")
----------------------------------------------
 0. lr_in_admission (ovn-northd.c:9556): eth.dst == fa:16:3e:6b:1b:79 && inport == "lrp-7f82a5", priority 50, uuid ba5e360d
    xreg0[0..47] = fa:16:3e:6b:1b:79;
    next;
 1. lr_in_lookup_neighbor (ovn-northd.c:9635): 1, priority 0, uuid 75a8a812
    reg9[2] = 1;
    next;
 2. lr_in_learn_neighbor (ovn-northd.c:9644): reg9[2] == 1, priority 100, uuid 72706c4a
    next;
 3. lr_in_ip_input (ovn-northd.c:10884): ip6 && ip6.dst == 2a01:49a0:112a:: && !ip.later_frag, priority 70, uuid 13d497b1
    icmp6 { eth.dst <-> eth.src; ip6.dst <-> ip6.src; ip.ttl = 255; icmp6.type = 1; icmp6.code = 3; next; };

icmp6
-----
    eth.dst <-> eth.src;
    ip6.dst <-> ip6.src;
    ip.ttl = 255;
    icmp6.type = 1;
    icmp6.code = 3;
    next;
10. lr_in_ip_routing (ovn-northd.c:8710): ip6.dst == 2a01:49a0:112a::/112, priority 225, uuid 57af3cd5
10. lr_in_ip_routing (ovn-northd.c:8710): ip6.dst == 2a01:49a0:112a::/112, priority 225, uuid 57af3cd5
    ip.ttl--;
    reg8[0..15] = 0;
    xxreg0 = ip6.dst;
    xxreg1 = 2a01:49a0:112a::;
    eth.src = fa:16:3e:6b:1b:79;
    outport = "lrp-7f82a5";
    flags.loopback = 1;
    next;
11. lr_in_ip_routing_ecmp (ovn-northd.c:9902): reg8[0..15] == 0, priority 150, uuid 7e1242f0
    next;
12. lr_in_policy (ovn-northd.c:10027): 1, priority 0, uuid dfb65200
    reg8[0..15] = 0;
    next;
13. lr_in_policy_ecmp (ovn-northd.c:10029): reg8[0..15] == 0, priority 150, uuid a59e560f
    next;
14. lr_in_arp_resolve (ovn-northd.c:10245): outport == "lrp-7f82a5" && xxreg0 == 2a01:49a0:112a::289, priority 100, uuid 1e99a10c
    eth.dst = fa:16:3e:f5:aa:2a;
    next;
18. lr_in_arp_request (ovn-northd.c:10652): 1, priority 0, uuid d05b7ef1
    output;

egress(dp="test-router", inport="lrp-7f82a5", outport="lrp-7f82a5")
-------------------------------------------------------------------
 3. lr_out_delivery (ovn-northd.c:10700): outport == "lrp-7f82a5", priority 100, uuid 3d0b42d6
    output;
    /* output to "lrp-7f82a5", type "patch" */

ingress(dp="piotr-geneve-ipv6-stateful", inport="7f82a5")
---------------------------------------------------------
 0. ls_in_port_sec_l2 (ovn-northd.c:5023): inport == "7f82a5", priority 50, uuid e1f011ad
    next;
 5. ls_in_pre_acl (ovn-northd.c:5166): ip && inport == "7f82a5", priority 110, uuid f4b6d9ab
    next;
 6. ls_in_pre_lb (ovn-northd.c:5166): ip && inport == "7f82a5", priority 110, uuid c42a6170
    next;
 8. ls_in_acl_hint (ovn-northd.c:5486): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0, priority 4, uuid 5d064a89
    reg0[8] = 1;
    reg0[10] = 1;
    next;
23. ls_in_l2_lkup (ovn-northd.c:7615): eth.dst == fa:16:3e:f5:aa:2a, priority 50, uuid 7faa449b
    outport = "9b6d8c";
    output;

egress(dp="piotr-geneve-ipv6-stateful", inport="7f82a5", outport="9b6d8c")
--------------------------------------------------------------------------
 1. ls_out_pre_acl (ovn-northd.c:5226): ip, priority 100, uuid 32588e71
    reg0[0] = 1;
    next;
 2. ls_out_pre_stateful (ovn-northd.c:5411): reg0[0] == 1, priority 100, uuid 58780681
    ct_next;

ct_next(ct_state=est|trk /* default (use --ct to customize) */)
---------------------------------------------------------------
 4. ls_out_acl_hint (ovn-northd.c:5486): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0, priority 4, uuid 99195420
    reg0[8] = 1;
    reg0[10] = 1;
    next;
 5. ls_out_acl (ovn-northd.c:5744): reg0[10] == 1 && (outport == @neutron_pg_drop && ip), priority 2001, uuid ec8b7afe
    ct_commit { ct_label.blocked = 1; };