examples/ipsec-secgw: support inline protocol

examples/ipsec-secgw: support inline protocol

Adding support for inline protocol processing

In ingress side, application will receive regular IP packets, without
any IPsec related info. Application will do a selector check (SP-SA
check) by making use of the metadata from the packet. The
device-specific metadata in mbuf would aid in determing the security
session which processed the packet.

In egress side, the plain packet would be submitted to the driver. The
packet will have optional metadata, which could be used to identify the
security session associated with the packet.

Signed-off-by: Anoob Joseph <anoob.joseph@caviumnetworks.com>
Reviewed-by: Akhil Goyal <akhil.goyal@nxp.com>