examples/ipsec-secgw: fix wrong session size

examples/ipsec-secgw: fix wrong session size

Crypto devices, which support lookaside protocol, exposes security
session size in addition to the crypto private symmetric session data
size. For applications using the security capabilities, both these
sizes need to be considered.

Fixes: ec17993a145a ("examples/ipsec-secgw: support security offload")
Cc: stable@dpdk.org
Signed-off-by: Anoob Joseph <anoob.joseph@caviumnetworks.com>
Signed-off-by: Archana Muniganti <muniganti.archana@caviumnetworks.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>