app/testpmd: fix reply to a multicast ICMP request

app/testpmd: fix reply to a multicast ICMP request

Set the IP source and destination addresses in the IP header of the
ICMP reply as follows:
  - Use the request IP source address as the reply IP destination address
  - If the request IP destination address is a multicast IP address
      - choose a reply IP source address different from the request IP
        source address,
      - re-compute the IP header checksum.
    Otherwise
      - switch the request IP source and destination addresses in the
        reply,
      - keep the IP header checksum unchanged.

Signed-off-by: Ivan Boule <ivan.boule@6wind.com>