set packet_info->l2_vlan_header

packet_info->l2_vlan_header is referenced by set_vlan_vid(),
set_vlan_pcp() and execute_action_pop_vlan() but it is not set in
parse_ether()
Merge pull request #63 from iHiroakiKawai/l2_vlan_header

set packet_info->l2_vlan_header