examples/ipsec-secgw: fix IPv4 checksum at Tx
examples/ipsec-secgw: fix IPv4 checksum at Tx

For ESP transport and BYPASS mode the app might generate output
packets with invalid IPv4 header checksum.
At least such behavior was observed on few Intel NICs.
The reason is that the app didn't set ipv4 header checksum to zero
before passing it to the HW.

Fixes: 906257e965b7 ("examples/ipsec-secgw: support IPv6")
Cc: stable@dpdk.org

Signed-off-by: Konstantin Ananyev <cbd5212b59ab42a210d72992d23a3aa17cfd3eaf@intel.com>
Acked-by: Radu Nicolau <fa459cf59d47c122ba776e9a6f631c862bf44680@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk