vxlan: fix vxlan hw offload issue

vxlan: fix vxlan hw offload issue

Type: fix

Since Vxlan hw offload jumps the ethernet-input node, so needs to
adjust the data offset accordingly

In original code, the current_data is 0 when arriving vxlan-flow-input
node(due to no graph node before it, except the dpdk-input), so this
code block cannot find the correct vxlan header:
enum
  { payload_offset = sizeof (ip4_vxlan_header_t) };
vlib_buffer_advance (b0, payload_offset);
see code in src/vnet/vxlan/decap.c, function vxlan4_flow_input_node

This patch fixes this issue

Signed-off-by: Chenmin Sun <chenmin.sun@intel.com>
Change-Id: Iab4af7a7dc3b69a117a4c9ea1c59662669a6438c