examples/ipv4_multicast: support double vlan stripping and insertion
examples/ipv4_multicast: support double vlan stripping and insertion

The outer vlan should be copied from source packet buffer to
support double vlan stripping and insertion, as double vlan can be
stripped or inserted by some of NIC hardware.

Signed-off-by: Helin Zhang <aba9dfdb1a6ec8bf1ff66b95272c029f8dfde1b1@intel.com>
Acked-by: Jingjing Wu <64d28b617c3b4840f5ded1bfa611cba7fadfe5c7@intel.com>

msune/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,msune/dpdk,mixja/dpdk,mixja/dpdk,mixja/dpdk,venkynv/dpdk-mirror,tsphillips/dpdk-fork,venkynv/dpdk-mirror,mixja/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,msune/dpdk,msune/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk