examples/ipsec-secgw: fix usage of incorrect port

examples/ipsec-secgw: fix usage of incorrect port

When security offload is enabled, the packet should be forwarded on the
port configured in the SA. Security session will be configured on that
port only, and sending the packet on other ports could result in
unencrypted packets being sent out.

This would have performance improvements too, as the per packet LPM
lookup would be avoided for IPsec packets, in inline mode.

Fixes: ec17993a145a ("examples/ipsec-secgw: support security offload")

Signed-off-by: Anoob Joseph <anoob.joseph@caviumnetworks.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>