examples/ipsec-secgw: move IV to crypto op private data

Usually, IV will change for each crypto operation.
Therefore, instead of pointing at the same location,
IV is copied after each crypto operation.

This will let the IV to be passed as an offset from
the beginning of the crypto operation, instead of
a pointer.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>
Acked-by: Fiona Trahe <fiona.trahe@intel.com>