[9.x swarm] Unknown overhead of package size for env with dpdk+vxlan

Detailed bug description:

The issue was found by https://product-ci.infra.mirantis.net/job/9.x.system_test.ubuntu.ovs_firewall_with_dpdk/1/testReport/(root)/deploy_ovs_firewall_and_dpdk_vxlan_ipv6/

Steps to reproduce:
            1. Create new environment with VxLAN segmentation for Neutron
            2. Add controller and compute nodes
            3. Enable OVS firewall driver for neutron security groups
            4. Configure private network in DPDK mode
            5. Configure HugePages for compute nodes
            6. Run network verification
            7. Deploy environment
            8. Run OSTF
            9. Boot instance
            10. Assign floating ip
            11. Ping instance:
                # ping -q -c 10 -w 40 -s 1423 <instance_floating_ip>

Expected results: Ping passes
because overhead should be

Outer IPv4 Header ----------- 20 bytes
Outer UDP Header ------------ 8 bytes
VXLAN Header ---------------- 8 bytes
----------------------------------------
                              36 bytes
Thus, if original MTU is 1500, then all packages with size < 1464 should pass
Actual result: Ping passes only for package size =< 1422
So, unknown overhead is 42 bytes